Include SHA of Target Branch HEAD for Failed Backports

[dsanders11]

There are from time to time trop failures to backport which when manually performed it is unclear why it failed since the cherry pick runs clean.

Would be nice if the backport failure check included the SHA for the target branch HEAD at the time it attempted the backport, to make it easier to debug these failures.

Output could look like:

---

Backport Failed

This PR was checked and could not be automatically backported to 23-x-y@e508ef2 cleanly