Improve visibility of background migration failures

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I developed a new background migration, and a few jobs had failed, but I was unaware of it for quite a while. Not sure if I missed something obvious but I couldn't see abnormalities on grafana or kibana. If we can increase the visibility of background migration errors, or make it clearer on how to monitor this in the handbook, it would be great :)

One suggestion is to have a slack bot for pinning MR author if things got wrong: https://gitlab.com/gitlab-org/gitlab-ce/issues/58213