Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/281835

**Problem**

Pull mirroring jobs retry to update mirrors even for hard failures
until they reach a retry limit. When a big external git hosting has a
persistent problem, then requests generated by update mirror jobs can
violate the acceptable error ratio and trigger an incident.

**Solution**

Identify unrecoverable failures and set a hard failure state for update
mirror jobs. Trigger an email to the customer notifying them about the
problem.