restricted_path: add option to diagnose as a warning instead of an error

Rather than a hard error we may for some projects just want to call attention to the fact that a MR modifies a path that is restricted in some way.

Also, the path is restricted message may need alternative wording for the warning. We could also add a placeholder for configuration of a custom segment of the message explaining why it is restricted and where the change should be made instead (similar to third_party).