Improve handling of private GitHub repos

[david-a-wheeler]

If someone adds a GitHub repo that's private, we handle it correctly externally but through exception handlers. This creates absurdly long log entries like this:

Jan 01 22:20:13 production-bestpractices app/web.1: ERROR:: In method propose_one_change, exception no implicit conversion of String into Integer on RepoFilesExamineDetective, current_proposal= {:

We don't need all that logging for this case; simplify things. We could return more information in this case.