Adding custom error messages to git responses

[azabujuban]

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In our organization, we have enabled mandatory GPG signatures for commits. We created a walk-through for users to enable it and put up a banner on the organization page.

Still, not everybody noticed it and when users try to push unsigned commits, they are presented with a generic error message (error: GH013: Repository rule violations found for refs/heads/master).

This leads to some wasted time to research the error message and check internally what needs to be done. Since the error happens during push it's not immediately obvious that you should open the organization GH page and search for instructions there.

It would be nice to allow some customization of error messages returned to git. This way it would be easy to put a link to the instructions for solving this or other errors.