ws: Unhandled exceptions from SocketClient

[etegan]

Describe the issue
The SocketClient._connect() function sets up a bunch of StreamSubscriptions on _messages that don't have any onError callbacks. For example #1228 will result in multiple unhandled exceptions that can't be caught by the user of this package.

To Reproduce

1. Make GraphQLSocketMessage.parse throw any exception
2. Try to catch it from any application that tries to create a ws connection
3. observe the mayhem
4. (optional) Change Stream<GraphQLSocketMessage> get _messages => socketChannel!.messages; in SocketClient
   to Stream<GraphQLSocketMessage> get _messages => socketChannel!.messages.handleError((e) => print("Getting errors ${e}"))

Expected behavior
Exceptions from SocketClient should not escape _connect() or the user should be able to catch them.

[vincenzopalazzo]

You are right, there is some work to do to modify the error handling!

Thanks to report it