Provide flag to fail if uncovered dependencies found

[hugochinchilla]

Hello, I'm using this on a gitlab pipeline to reject code which doesn't follow the dependency rules, it works great, thanks for this wonderful piece of software.

I would like to be able to be a bit more strict about breaking the rules and there is a case where new external dependencies can be added to the codebase and if not added to the list of vendor namespaces in the depfile they can be used outside the allowed boundaries without any way to detect it from the pipeline.

I would like to have the option to pass an extra flag like --fail-on-uncovered which will make the program to exit with an status different than 0 if any uncovered cases are found.