Add an option to convert warnings to errors

[marekdedic]

Hi,
since I am using this in an automated way, I'd like to be able to enforce a kind of strict mode, where the action fails if there are any errors or warnings, could that be added as an option?

[swissspidy]

We recently added severity levels in the plugin, which we can expose in the GitHub Action as well to make this possible. Makes sense to me 👍

[swissspidy]

In addition to that, I think I'd still need to change the exit code accordingly to make it truly strict.