Notify the user who set auto-merge when a build fails

This is really annoying:

• I review an MR - LGTM! 🎉
• I edit the commit message and click 'Merge when build succeeds' 💚
• I go about my day.
• The build fails.
• I don't get notified, and the MR doesn't get merged.

Not-great solutions:

• Add a todo for the MR, so that I can easily see if it's been merged when going through my todos.
• Keep the tab open, so I get a notification when the build fails.

I think for now I'd just like to get a todo when that happens.

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/22236

/cc @ayufan