Assign issue reports success but silently does nothing when the assignee doesn't exist#527

[WillGardella]

Was https://app.zenhub.com/workspace/o/atomisthq/bot-service/issues/527 but moved here

Easily reproducible. Pick a non-existent GitHub user like AlbertFustilugs and the bot will tell you it did the work, but it is a terrible liar and if you go to the issue it will have no assignee.