Add PR comment if automerge attempt fails

[rarkins]

Describe the proposed change(s).

If Renovate attempts a PR automerge, and the attempt fails (e.g. is rejected by the platform), then Renovate should add a PR comment with information about the failure. This comment should remain in place indefinitely, either as-is or potentially being updated if the failure reason changes.

Implementation suggestion:

1. Add a new enum configuration option automergeFailureComment, which defaults to "never", and also accepts "on-error".
2. Update the tryPrAutomerge() function to return a result, either:
   • Successful
   • Unsuccessful with a description (which may include logs)
3. If unsuccessful, the worker layer will then check for the value of automergeFailureComment. If set to "never", no comment is added (same as today's functionality). If it's set to "on-error" then the worker should ensure a PR comment with title "Automerge failure notification" and include the failure description passed back from the platform.