Mirror silently fails to update if diverged from upstream
Zendesk issue: https://gitlab.zendesk.com/agent/tickets/14885
The mirror repository feature will silently fail to update master branch if the mirror's master branch has diverged from the upstream master. It makes sense that we cannot update the mirror in this case, but we need to let the user know.
The silent failure is occurring at https://gitlab.com/gitlab-org/gitlab-ee/blob/master/app%2Fservices%2Fprojects%2Fupdate_mirror_service.rb#L41-42
cc/ @DouweM Can we alert the user in this case?