Send an email to project owners when a mirror update fails
Currently mirror update failures are somewhat silent. The user has to go to the project or mirror settings page and check that an error has occurred. This reduced visibility affects the GitLab team as well, since mirroring is an important part of our infrastructure process (see https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5313). Adding an email alert similar to the one a user receive when a pipeline fails would solve this issue.