Feature: Exit non-zero with -0 on 'No packages'

[davetapley]

Before You Begin

Before proceeding, please make sure to follow these steps:

• I have checked for similar feature requests in the project's issue tracker
  to avoid duplicates.
• I have searched existing issues to see if this feature has been discussed
  before.

Feature Details

If someone is using this package, presumably they have at least one package they want to check, so No packages probably indicates misconfiguration and exiting non-zero with -0 would be less surprising.

Is your feature request related to a problem? Please describe

I would have missed this if I hadn't 👀 on CI logs:

• Question: Use with GitHub Actions and uv #131

Describe the solution you'd like

Fail CI.

Describe alternatives you've considered

Manually 👀 on CI logs to check it actually ran 🤢