feat: use return error codes for easier integration in other projects

[BoboTiG]

That way we can use that cool new tool into our automation without having to parse the STDIN output.

[tserg]

Thanks for this. I don't foresee any integration for the moment, so I will just leave this PR open until the need arises.

[BoboTiG]

I mean for now if I want to use Medusa in my projects, like I would do with flake8, I have to parse the output to be sure there was no issues. With that PR, I will know something "bad" was found, or not, thanks to the return code (it will stop the process with an error for instance).