[GHF] Print better merge rule match reason
When running `find_matching_merge_rule` raise an exception that:
- Contains generic message if files changed do not match any rules
- Mentions rule that matches majority of files, but calls out few
non-matching ones
- Mentions matching rules, but calls out that it lacks reviewers
- Mentions rule, but calls out missing mandatory checks
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74887
Approved by: https://github.com/osalpekar, https://github.com/janeyx99
