fix(core): Fix crash when no negotiator matches

Fixes #30.

Also removes the `allowPlainAuth` attribute of `ConnectionSettings` as
users who want to disable SASL PLAIN can just not register the
negotiator or extend it.

packages/moxxmpp/lib/moxxmpp.dart

@@ -1,6 +1,7 @@
 library moxxmpp;
 
 export 'package:moxxmpp/src/connection.dart';
+export 'package:moxxmpp/src/connection_errors.dart';
 export 'package:moxxmpp/src/connectivity.dart';
 export 'package:moxxmpp/src/errors.dart';
 export 'package:moxxmpp/src/events.dart';