Cope with config errors when running `is_used`

[nathanjmcdougall]

We already cope with decoding errors (e.g. syntax errors) but not general configuration errors (e.g. schema violations).

This came up in https://github.com/usethis-python/usethis-python/actions/runs/15938447778/job/44962804883?pr=813

[nathanjmcdougall]

I don't think config errors are actually possible in this case anyway, since the only schemas we have are for pre-commit and bitbucket pipelines, neither of which are checked in is_used, so this is more just for future robustness when working on #811