ActivityQueue: Notify Sentry server when you can't deliver an activity

Currently when the sender is unable to deliver an activity to a receiver, it will try multiple times, but at some point it has to give up. At this point, it simply logs an error to the djangoldp logger

It would be better, if a sentry server is configured (#273 (closed) ), to send a notification here, and alert the sysadmin of the inconsistency in the federation

We may be able to do some magic to detect the sentry server without settings.py