Update exception handling

Allow WebSocketHandler methods to raise an exception.

By default we install ExceptionWebSocketHandlerDecorator, which logs
unhandled exceptions and closes the session. That decorator can be
extended or replaced.

Any exceptions that remain unhandled still (i.e. no exception handling
decorator), are caught in the lowest level before propagating to the
WebSocket engine or a SockJS transport handler and handled the same
way. That means default behavior is guaranteed but also fully
customizable.