Propagate exceptions from a message handler

[saaj]

BaseActor._task should call set_exception on the Future of QueryMessage when message handler raises. It can look like this:

    @asyncio.coroutine
    def _task(self):
        message = yield from self._inbox.get()
        try:
            handler  = self._handlers[type(message)]
            is_query = isinstance(message, QueryMessage)
            try:
                response = yield from handler(message)
            except Exception as ex:
                if is_query:
                    message.result.set_exception(ex)
            else:
                if is_query:
                    message.result.set_result(response)
        except KeyError as ex:
            raise HandlerNotFoundError(type(message)) from ex

What I'm not sure about is whether plain message handler's exception should be re-risen, eaten or maybe a warning should be emitted.