Ability to highlight author if a backport PR cannot be created

[leseb]

If Mergify fails to cherry-pick commits and thus to create a backport PR, it would be nice to highlight the author about that.

Really similar to the recent conflict action.

Thanks.

[jd]

It'd be cool to be able to do something like:

- name: notify the author if backport fails
  conditions:
    - label=conflicts
    # Optionally match on the branch name here
  actions:
    comment:
      message: This backport is in conflict @{{backported.author}}

That would require Mergify to track the PR it has backported, but that shouldn't be too hard.