config.types.ChoiceAttribute: log the invalid value if validation fails

[dgw]

Description

This should make debugging a bad configuration easier, especially if the value is read from an environment variable.

It should also help with figuring out weird errors related to invisible characters in the configuration file, but that's not the primary motivation here.

Checklist

• [x] I have read CONTRIBUTING.md
• [x] I can and do license this contribution under the EFLv2
• [x] No issues are reported by make qa (runs make lint and make test)
• [x] I have tested the functionality of the things this change touches

[dgw]

Could go in Sopel 8.0.1.

It could. Easy cherry-pick, so why not.