Feature Request: Considering handling a SkipEnqueue MiddlewareError

I've just implemented a "DedupMiddleware" which can be used prevent a message from being enqueued if a similar message already exists in the queue. Unfortunately it requires to override RedisBroker.enqueue() in order to gracefully catch the SkipEnqueue middleware error I raise when needed.

Would you consider accepting a PR that would:

1. Define a SkipEnqueue(MiddlewareError).
2. try...except this error around self.emit_before here, so a middleware could be given the possibility to interrupt an enqueuing in that hook?

Thanks.