Ability to retry a message with custom exceptions

[EmilioCarrion]

It would be useful to explicitly retry a message without the need of raising an exception. We currently have this behaviour:

try:
    res = self._subscription(data, **dict(message.attributes))
except Exception as e:
    run_middleware_hook(
        "post_process_message_failure",
        self._subscription,
        e,
        start_time,
        message,
    )
else:
    message.ack()
    run_middleware_hook(
        "post_process_message_success",
        self._subscription,
        start_time,
        message,
    )
    return res
finally:
    run_middleware_hook("post_process_message")

When the message is acked only when an exception is not raised. As commented in #80

It would be useful to be able to ack or retry the message explicitly using custom exceptions, per example:

raise Retry() (that would do nothing or a nack, this is discussed in the referenced issue)
raise Discard() (that would execute a message.ack())