Fixed exception. Add onError stream for server ERROR frames.

[Dumbris]

1) Fixed "Exception: Bad state: Future already completed Stack Trace:

0 _AsyncCompleter.completeError (dart:async/future_impl.dart:35)

1 Client.connect. (package:stompdart/stomp.dart:224:36)

..."

caused by any ERROR message after connect.

2) Add onError stream to Client class. Error handling depends on user.

[michaelkrog]

Looks great. Merging in.