Cherrypicker: create GH issue on failed cherrypick

[clarketm]

What would you like to be added:

Due to merge conflicts, it is inevitable that a cherrypick will fail.

e.g.: istio/istio#20949

While the above GitHub comment reporting on the PR it helpful, it would be beneficial to additionally create a GitHub issue (assigned to the PR author) as a reminder that the cherrypick failed and needs to be performed manually.

We often see cherrypicks fail and devs don't notice. It would be nice to create an issue for failed cherrypicks, that way they can be easily tracked. Bonus points if it sets the milestone and priority, but those are Istio specific so that may not be feasible
– @howardjohn (istio/test-infra#2423)

Why is this needed:
Convenience and user experience (UX).

/assign