Adding a new Rx BusNotifications API

The first cab of the rank is to expose error streams:
- MessageHasFailedAFirstLevelRetryAttempt
- MessageHasBeenSentToSecondLevelRetries
- MessageSentToErrorQueue
We can make those public sets since I'm publishing copies.
My only concern is whether when we publish this data and one of the subscribers changes it, will the other subscribers get those changes too? //cc @distantcam