Surface via output if a backport PR has already been merged when trying to backport

[codebytere]

As in title. Example: electron/electron#41793 showed needs-manual as a result of another bug but when I tried to run backports via trop again, it failed with blank diff instead of surfacing that the labels were just wrong and it'd already been merged into release branches: electron/electron#41805

[dsanders11]

Related to #252 - in general if any error happens the only thing visible to end users is a comment that it failed and a blank diff, so we need a general way to communicate an error occurred to end users instead of a confusing blank diff.