Only expose one error type in crate root

tokio-xmpp/ChangeLog

@@ -5,6 +5,8 @@ XXXX-YY-ZZ RELEASER <admin@example.com>
       - Removed StreamFeatures from this crate, replaced with xmpp_parsers::stream_features::StreamFeatures (!400)
       - `starttls::error::ConnectorError` variants have been merged with `starttls::error::Error`, except `ConnectorError::AllFailed`
         which was not used and has been completely removed (!418)
+      - `ProtocolError` and `AuthError` are no longer exported in crate root;
+        access them from `error` module (!423)
 
 Version 4.0.0:
 2024-07-26 Maxime “pep” Buquet <pep@bouah.net>