network: Move errors to error submodule

diff --git a/network/src/lib.rs b/network/src/lib.rs
index 395f4e46..16b80ad6 100644
--- a/network/src/lib.rs
+++ b/network/src/lib.rs
@@ -27,6 +27,10 @@ use internals::error::InputString;
 #[cfg(feature = "serde")]
 use serde::{de::Visitor, Deserialize, Deserializer, Serialize, Serializer};
 
+#[rustfmt::skip]                // Keep public re-exports separate.
+#[doc(no_inline)]
+pub use self::error::ParseNetworkError;
+
 /// What kind of network we are on.
 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
@@ -238,23 +242,6 @@ pub mod as_core_arg {
     }
 }
 
-/// An error in parsing network string.
-#[derive(Debug, Clone, PartialEq, Eq)]
-#[non_exhaustive]
-pub struct ParseNetworkError(InputString);
-
-impl fmt::Display for ParseNetworkError {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        // Outputs 'failed to parse <input string> as network'.
-        write!(f, "{}", self.0.display_cannot_parse("network"))
-    }
-}
-
-#[cfg(feature = "std")]
-impl std::error::Error for ParseNetworkError {
-    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
-}
-
 impl FromStr for Network {
     type Err = ParseNetworkError;
 
@@ -286,6 +273,30 @@ impl<'a> Arbitrary<'a> for NetworkKind {
     }
 }
 
+/// Error types for the network.
+pub mod error {
+    use core::fmt;
+
+    use internals::error::InputString;
+
+    /// An error in parsing network string.
+    #[derive(Debug, Clone, PartialEq, Eq)]
+    #[non_exhaustive]
+    pub struct ParseNetworkError(pub(super) InputString);
+
+    impl fmt::Display for ParseNetworkError {
+        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+            // Outputs 'failed to parse <input string> as network'.
+            write!(f, "{}", self.0.display_cannot_parse("network"))
+        }
+    }
+
+    #[cfg(feature = "std")]
+    impl std::error::Error for ParseNetworkError {
+        fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     #[cfg(feature = "std")]