Refactor cask audit warnings.

[reitermarkus]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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Previously cask audit would fail on both warnings and errors. This makes it so it only fails on errors. For this, some warnings are now hard errors (no casks should be failing these anyways).

This allows showing a warning for existing casks and hard errors for new casks, e.g. for the desc stanza. This way contributors will hopefully notice that we want a description to be added while not making CI fail for simple version bumps.

[claui]

LGTM