[FEATURE] Add failed backport label when backport fails

[dbwiddis]

Is your feature request related to a problem?

Sometimes backports fail.

Sometimes we miss this fact, see #875.

We should add a label so we can find these more easily.

What solution would you like?

Whatever OpenSearch core project does.

What alternatives have you considered?

Maintainers who merge should be castigated if they don't make sure the backport is generated. :)