Subscriber error handler

Brief rundown:

* Add a default ctor for the TinyMessengerHub, which uses an internal DefaultSubscriberErrorHandler that swallows any error in the subscriber code (same as previously)
* Add a ctor for TinyMessengerHub that allows injection of an ISubscriberErrorHandler class, so any errors occurring in the subscriber code can be handled how ever we wish
* Add a test to confirm behavior

All changes should be non-breaking.  Confirmed all existing tests are still green.