Add custom exception classes for exceptions that should trigger unroutable message replies and use source and destination setting functions to properly route replies.
Explicitly mark unroutable replies and special case events for them.
Add tests for events for unroutable replies that made it at least part way through a routing table.
Add tests for get_ and set_unroutable_reply.
Tests for unroutable event routing metadata helpers.
Remove errant apostrophe.
Fix typo in event processing comment.
Add test for message without tag or account.
Fix other typo in event routing comment.
Add test for outbound message with no target.
Add test for event whose outbound message has hops but no associated user account.
Add comments explaining which hops unroutable_event_done considers equal.
Merge branch 'feature/issue-983-fix-routing-events-for-replies-to-unroutable-messages' into develop