askrene: make failure messages clearer:

diff --git a/plugins/askrene/child/explain_failure.c b/plugins/askrene/child/explain_failure.c
index d97ecd01..364e41d0 100644
--- a/plugins/askrene/child/explain_failure.c
+++ b/plugins/askrene/child/explain_failure.c
@@ -137,7 +137,7 @@ static const char *check_capacity(const tal_t *ctx,
 	if (amount_msat_greater(amount, stats.total.capacity)) {
 		return child_log(ctx, LOG_DBG,
 				 NO_USABLE_PATHS_STRING
-				 "  Total %s capacity is only %s"
+				 " Total %s capacity is only %s"
 				 " (in %zu channels).",
 				 name,
 				 fmt_amount_msat(tmpctx, stats.total.capacity),
@@ -146,7 +146,7 @@ static const char *check_capacity(const tal_t *ctx,
 	if (amount_msat_greater(amount, stats.gossip_known.capacity)) {
 		return child_log(ctx, LOG_DBG,
 				 NO_USABLE_PATHS_STRING
-				 "  Missing gossip for %s: only known %zu/%zu channels, leaving capacity only %s of %s.",
+				 " Missing gossip for %s: only known %zu/%zu channels, leaving capacity only %s of %s.",
 				 name,
 				 stats.gossip_known.num_channels,
 				 stats.total.num_channels,
@@ -154,12 +154,20 @@ static const char *check_capacity(const tal_t *ctx,
 				 fmt_amount_msat(tmpctx, stats.total.capacity));
 	}
 	if (amount_msat_greater(amount, stats.enabled.capacity)) {
+		/* Common case: one channel, disabled */
+		if (stats.enabled.num_channels == 0) {
+			return child_log(ctx, LOG_DBG,
+					 NO_USABLE_PATHS_STRING
+					 " All %zu channels to the %s are disabled.",
+					 stats.total.num_channels,
+					 name);
+		}
 		return child_log(ctx, LOG_DBG,
 				 NO_USABLE_PATHS_STRING
-				 "  The %s has disabled %zu of %zu channels, leaving capacity only %s of %s.",
-				 name,
+				 " %zu of %zu channels to %s are disabled, leaving capacity only %s of %s.",
 				 stats.total.num_channels - stats.enabled.num_channels,
 				 stats.total.num_channels,
+				 name,
 				 fmt_amount_msat(tmpctx, stats.enabled.capacity),
 				 fmt_amount_msat(tmpctx, stats.total.capacity));
 	}
@@ -272,7 +280,7 @@ const char *explain_failure(const tal_t *ctx,
 			    fmt_amount_msat(tmpctx, rolling_amount));
 			return child_log(ctx, LOG_INFORM,
 				      NO_USABLE_PATHS_STRING
-					 "  The shortest path is %s, but %s %s",
+					 " The shortest path is %s, but %s %s",
 					 path,
 					 fmt_short_channel_id_dir(tmpctx, &scidd),
 					 explanation);
@@ -313,7 +321,7 @@ const char *explain_failure(const tal_t *ctx,
 
 		return child_log(ctx, LOG_INFORM,
 				 NO_USABLE_PATHS_STRING
-				 "  The shortest path is %s, but %s %s",
+				 " The shortest path is %s, but %s %s",
 				 path,
 				 fmt_short_channel_id_dir(tmpctx, &scidd),
 				 explanation);
diff --git a/tests/test_askrene.py b/tests/test_askrene.py
index 1a655bbf..53965fab 100644
--- a/tests/test_askrene.py
+++ b/tests/test_askrene.py
@@ -81,7 +81,7 @@ def test_reserve(node_factory):
     time.sleep(2)
 
     # Reservations can be in either order.
-    with pytest.raises(RpcError, match=rf'We could not find a usable set of paths.  The shortest path is {scid12}->{scid23}, but {scid12dir} already reserved 10000000*msat by command ".*" \([0-9]* seconds ago\), 10000000*msat by command ".*" \([0-9]* seconds ago\)'):
+    with pytest.raises(RpcError, match=rf'We could not find a usable set of paths. The shortest path is {scid12}->{scid23}, but {scid12dir} already reserved 10000000*msat by command ".*" \([0-9]* seconds ago\), 10000000*msat by command ".*" \([0-9]* seconds ago\)'):
         l1.rpc.getroutes(source=l1.info['id'],
                          destination=l3.info['id'],
                          amount_msat=1000000,
@@ -692,7 +692,7 @@ def test_getroutes(node_factory):
 
     # Too much should give a decent explanation.
     dir01 = direction(nodemap[0], nodemap[1])
-    with pytest.raises(RpcError, match=rf"We could not find a usable set of paths\.  The shortest path is 0x1x0, but 0x1x0/{dir01} isn't big enough to carry 1000000001msat\."):
+    with pytest.raises(RpcError, match=rf"We could not find a usable set of paths\. The shortest path is 0x1x0, but 0x1x0/{dir01} isn't big enough to carry 1000000001msat\."):
         l1.rpc.getroutes(source=nodemap[0],
                          destination=nodemap[1],
                          amount_msat=1000000001,
@@ -701,7 +701,7 @@ def test_getroutes(node_factory):
                          final_cltv=99)
 
     # This should tell us source doesn't have enough.
-    with pytest.raises(RpcError, match=r"We could not find a usable set of paths\.  Total source capacity is only 1019000000msat \(in 3 channels\)\."):
+    with pytest.raises(RpcError, match=r"We could not find a usable set of paths\. Total source capacity is only 1019000000msat \(in 3 channels\)\."):
         l1.rpc.getroutes(source=nodemap[0],
                          destination=nodemap[1],
                          amount_msat=2000000001,
@@ -710,7 +710,7 @@ def test_getroutes(node_factory):
                          final_cltv=99)
 
     # This should tell us dest doesn't have enough.
-    with pytest.raises(RpcError, match=r"We could not find a usable set of paths\.  Total destination capacity is only 1000000000msat \(in 1 channels\)\."):
+    with pytest.raises(RpcError, match=r"We could not find a usable set of paths\. Total destination capacity is only 1000000000msat \(in 1 channels\)\."):
         l1.rpc.getroutes(source=nodemap[0],
                          destination=nodemap[4],
                          amount_msat=1000000001,
@@ -724,7 +724,7 @@ def test_getroutes(node_factory):
     l1.rpc.askrene_update_channel(layer="chans_disabled",
                                   short_channel_id_dir=f'0x1x0/{dir01}',
                                   enabled=False)
-    with pytest.raises(RpcError, match=rf"We could not find a usable set of paths\.  The shortest path is 0x1x0, but 0x1x0/{dir01} marked disabled by layer chans_disabled\."):
+    with pytest.raises(RpcError, match=rf"We could not find a usable set of paths\. The shortest path is 0x1x0, but 0x1x0/{dir01} marked disabled by layer chans_disabled\."):
         l1.rpc.getroutes(source=nodemap[0],
                          destination=nodemap[1],
                          amount_msat=1000,
@@ -1390,7 +1390,7 @@ def test_max_htlc(node_factory, bitcoind):
                                   amount_msat=1,
                                   inform='constrained')
 
-    with pytest.raises(RpcError, match=rf"We could not find a usable set of paths.  The shortest path is 0x1x0, but 0x1x0/{dir01} exceeds htlc_maximum_msat ~1000448msat"):
+    with pytest.raises(RpcError, match=rf"We could not find a usable set of paths. The shortest path is 0x1x0, but 0x1x0/{dir01} exceeds htlc_maximum_msat ~1000448msat"):
         l1.rpc.getroutes(source=nodemap[0],
                          destination=nodemap[1],
                          amount_msat=20_000_000,
@@ -1424,7 +1424,7 @@ def test_min_htlc_after_excess(node_factory, bitcoind):
     l1 = node_factory.get_node(gossip_store_file=gsfile.name)
 
     dir01 = direction(nodemap[0], nodemap[1])
-    with pytest.raises(RpcError, match=rf"We could not find a usable set of paths.  The shortest path is 0x1x0, but 0x1x0/{dir01} below htlc_minumum_msat ~2000msat"):
+    with pytest.raises(RpcError, match=rf"We could not find a usable set of paths. The shortest path is 0x1x0, but 0x1x0/{dir01} below htlc_minumum_msat ~2000msat"):
         l1.rpc.getroutes(source=nodemap[0],
                          destination=nodemap[1],
                          amount_msat=1999,
@@ -1435,11 +1435,11 @@ def test_min_htlc_after_excess(node_factory, bitcoind):
 
 # These were obviously having a bad day at the time of the snapshot:
 canned_gossmap_badnodes = {
-    19: "We could not find a usable set of paths.  The shortest path is 103x1x0->0x2134x0->988x333x988->16188x333x16169, but 0x2134x0/0 exceeds htlc_maximum_msat ~1000448msat",
-    53: "We could not find a usable set of paths.  The destination has disabled 177 of 177 channels, leaving capacity only 0msat of 4003677000msat.",
-    69: "We could not find a usable set of paths.  The destination has disabled 151 of 151 channels, leaving capacity only 0msat of 9092303000msat.",
-    72: "We could not find a usable set of paths.  The destination has disabled 146 of 146 channels, leaving capacity only 0msat of 1996000000msat.",
-    86: "We could not find a usable set of paths.  The destination has disabled 131 of 131 channels, leaving capacity only 0msat of 162000000msat.",
+    19: "We could not find a usable set of paths. The shortest path is 103x1x0->0x2134x0->988x333x988->16188x333x16169, but 0x2134x0/0 exceeds htlc_maximum_msat ~1000448msat",
+    53: r"We could not find a usable set of paths\. All 177 channels to the destination are disabled\.",
+    69: r"We could not find a usable set of paths\. All 151 channels to the destination are disabled\.",
+    72: r"We could not find a usable set of paths\. All 146 channels to the destination are disabled\.",
+    86: r"We could not find a usable set of paths\. All 131 channels to the destination are disabled\.",
 }
 
 
@@ -2055,7 +2055,7 @@ def test_askrene_reserve_clash(node_factory, bitcoind):
                                   }])
 
     # We can't use this on layer 1 anymore, only 50000 msat left.
-    with pytest.raises(RpcError, match=r"We could not find a usable set of paths.  The shortest path is 0x0x0, but 0x0x0/1 already reserved 950000msat by command"):
+    with pytest.raises(RpcError, match=r"We could not find a usable set of paths. The shortest path is 0x0x0, but 0x0x0/1 already reserved 950000msat by command"):
         l1.rpc.getroutes(source=l1.info['id'],
                          destination=node1,
                          amount_msat=500000,
@@ -2609,7 +2609,7 @@ def test_impossible_payment(node_factory):
     )
     with pytest.raises(
         RpcError,
-        match=r"We could not find a usable set of paths.  The shortest path is 0x0x1->0x0x2, but 0x0x1/0 exceeds htlc_maximum_msat",
+        match=r"We could not find a usable set of paths. The shortest path is 0x0x1->0x0x2, but 0x0x1/0 exceeds htlc_maximum_msat",
     ):
         l1.rpc.getroutes(
             source=node1,
@@ -2621,7 +2621,7 @@ def test_impossible_payment(node_factory):
         )
     with pytest.raises(
         RpcError,
-        match=r"We could not find a usable set of paths.  The shortest path is 0x0x1->0x0x2, but 0x0x1/0 exceeds htlc_maximum_msat",
+        match=r"We could not find a usable set of paths. The shortest path is 0x0x1->0x0x2, but 0x0x1/0 exceeds htlc_maximum_msat",
     ):
         l1.rpc.getroutes(
             source=node1,
diff --git a/tests/test_xpay.py b/tests/test_xpay.py
index 1f9106e1..d4eeef6a 100644
--- a/tests/test_xpay.py
+++ b/tests/test_xpay.py
@@ -3,7 +3,7 @@ from fixtures import TEST_NETWORK
 from pyln.client import RpcError
 from pyln.testing.utils import FUNDAMOUNT, only_one
 from utils import (
-    TIMEOUT, first_scid, GenChannel, generate_gossip_store, wait_for,
+    TIMEOUT, first_scid, first_scidd, GenChannel, generate_gossip_store, wait_for,
     sync_blockheight,
 )
 
@@ -197,11 +197,11 @@ def test_xpay_simple(node_factory):
 
     # Failure from l3 (with routehint)
     l4.stop()
-    with pytest.raises(RpcError, match=r"Failed after 1 attempts\. We got temporary_channel_failure for the invoice's route hint \([0-9x]*/[01]\), assuming it can't carry 10000msat\. Then routing failed: We could not find a usable set of paths\.  The shortest path is [0-9x]*->[0-9x]*->[0-9x]*, but [0-9x]*/[01]\ layer xpay-7 says max is 9999msat"):
+    with pytest.raises(RpcError, match=r"Failed after 1 attempts\. We got temporary_channel_failure for the invoice's route hint \([0-9x]*/[01]\), assuming it can't carry 10000msat\. Then routing failed: We could not find a usable set of paths\. The shortest path is [0-9x]*->[0-9x]*->[0-9x]*, but [0-9x]*/[01]\ layer xpay-7 says max is 9999msat"):
         l1.rpc.xpay(b11)
 
     # Failure from l3 (with blinded path)
-    with pytest.raises(RpcError, match=r"Failed after 1 attempts. We got an error from inside the blinded path 0x0x0/1: we assume it means insufficient capacity. Then routing failed: We could not find a usable set of paths.  The shortest path is [0-9x]*->[0-9x]*->0x0x0, but 0x0x0/1 layer xpay-8 says max is 99999msat"):
+    with pytest.raises(RpcError, match=r"Failed after 1 attempts. We got an error from inside the blinded path 0x0x0/1: we assume it means insufficient capacity. Then routing failed: We could not find a usable set of paths. The shortest path is [0-9x]*->[0-9x]*->0x0x0, but 0x0x0/1 layer xpay-8 says max is 99999msat"):
         l1.rpc.xpay(b12)
 
     # Restart, try pay already paid one again.
@@ -1074,6 +1074,23 @@ def test_xpay_blockheight_mismatch(node_factory, bitcoind, executor):
     fut.result(TIMEOUT)
 
 
+def test_error_messages(node_factory):
+    """Nicer error messages when we disable the only channel to the destination."""
+    plugin = os.path.join(os.path.dirname(__file__), 'plugins/replace_payload.py')
+    l1, l2 = node_factory.line_graph(
+        2,
+        opts=[{}, {"plugin": plugin}],
+        wait_for_announce=True
+    )
+
+    # Replace with an invalid payload.
+    l2.rpc.call('setpayload', ['0000'])
+    inv = l2.rpc.invoice(123, 'test_error_messages', 'test_error_messages')['bolt11']
+    scidd = first_scidd(l1, l2)
+    with pytest.raises(RpcError, match=rf"Failed after 1 attempts\. Unexpected error \(invalid_onion_payload\) from final node: disabling {scidd} for this payment\. Then routing failed: We could not find a usable set of paths\. All 1 channels to the source are disabled."):
+        l1.rpc.xpay(inv)
+
+
 def test_blinded_path_fees(node_factory):
     """Test that we don't send the amount+fees to our direct peer (we should
     only send the required amount) when the sending node is the entry point in