Fail loudly when auto-deploy pick isn't mirrored

Previously, if something was wrong with mirroring or merge-train and auto_deploy_on_security was enabled, we'd be unable to find the merge commit and would mark the pick as a failure, kicking it back to the author telling them something went wrong.

In reality, something was wrong that only the Delivery team should be responsible for, so we now raise a MirrorError and fail loudly. The merge request in question is not updated at all, so that we can resolve the error and re-run picking.