option to report unused dependencies in tach check-external as an error instead of a warning

[DetachHead]

> tach check-external
⚠️  Unused dependencies from project at 'pyproject.toml':
        nodejs_wheel

Remove the unused dependencies from the corresponding pyproject.toml file, or consider ignoring the dependencies by adding them to the 'external.exclude' list in tach.toml.

✅ All external dependencies validated!

this exits with exit code 0, but it would be nice if there was an option to report this as an error so exits with a non-zero exit code.