status_field_for_acks
It's been talked about before - but we should consider adding a status field (byte?) to acknowledgement messages.
The reason we would want this is that the current version silently ignores errors (don't have permission to publish/subscribe, etc.) Silent failures cannot be logged, reported or reacted to.
CONNACK already has this. It should be added to PUBACK, SUBACK and UNSUBACK.
