Catch Message.DoesNotExist error in message delete handler

[brizandrew]

If someone deletes a message in the database, and then deletes the message in Slack, Slack will send a message_delete event which will be handled. Unfortunately, if the message does not exist the handler error, responding to slack with a 500 error, which will make Slack retry the event repeatedly.

We need to catch the error in the handler, so that if the message does not exist, a 200 is returned anyway.