Implement std::error::Error for SocketParseError

SocketParseError does not implement std Error which means it can't be
used in standard ways like combining ? and anyhow.

This commit implements std::error::Error for SocketParseError

Hide the std::error::Error for SocketAddressParseError implementation
behind the "std" feature since it relies on the standard library.

Author: Anonymous

lightning/src/ln/msgs.rs

@@ -1078,6 +1078,9 @@ impl fmt::Display for SocketAddressParseError {
 	}
 }
 
+#[cfg(feature = "std")]
+impl std::error::Error for SocketAddressParseError {}
+
 #[cfg(feature = "std")]
 impl From<std::net::SocketAddrV4> for SocketAddress {
 	fn from(addr: std::net::SocketAddrV4) -> Self {