Pinpoint commits where error occurred

[sdemos]

This comes up in particularly large commits like the rust rewrites of coreos-metadata and update-ssh-keys, but I think giving more guidance as to which commit broke which rules would be pretty useful.

related, maybe there should be a separate error message for each commit which fails a particular check? I think that becomes a natural way to display the errors if they become commit-specific.

seems related to #25 but I thought I would add a new issue that was more specific.

[crawford]

I agree this would be great. I'm not sure how Tailor would determine this from it's expression language though. Maybe it would be possible to hold back-references to the underlying context where a rule failed.