`commenter add-loop` repeats itself after error?

I am not completely sure but it seems like maybe if `commenter add-loop` encounters an error/a problem while parsing build-constraints.yaml then it repeats existing constraints in its output.

I think one way to reproduce is to add some "nonsense" under `Stackage Upperbounds`.
eg I had `placeholder < -1` (a "double error": unknown package and bad upperbound)
and I believe this made `commenter add-loop` repeat endlessly afaict.

I would rather it just gave up with an error.