fix: Add missing re-export for TransportErrorCode

This is in the public interface, as a field of `Close::Transport`. Sadly
this isn't detected by the `unreachable_pub` lint.

src/lib.rs

@@ -68,7 +68,7 @@ pub use config::Config;
 pub use connection_pool::ConnId;
 pub use connections::{DisconnectionEvents, RecvStream, SendStream};
 pub use endpoint::{Endpoint, IncomingConnections, IncomingMessages};
-pub use error::{Close, ConnectionError, Error, Result};
+pub use error::{Close, ConnectionError, Error, Result, TransportErrorCode};
 
 #[cfg(test)]
 mod tests;