Fixed exception. Add onError stream for server ERROR frames.

1) Fixed
"Exception: Bad state: Future already completed
Stack Trace: 
#0      _AsyncCompleter.completeError (dart:async/future_impl.dart:35)
#1      Client.connect.<anonymous closure> (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.