files: wrong `field` for error messages

[slint]

We should review the field for the error at

invenio-drafts-resources/invenio_drafts_resources/services/records/components/files.py

Line 72 in 4684579

"field": "files.enabled",

(and possibly other errors) so that it properly reflects to the correct level. E.g. in this case it should just be field: files, since the error has to do with all of the files and not just the files.enabled attribute.

• Fix the field for all error messages
• Cross-check with the deposit UI in case the errors are rendered on a specific field/part of the form

Originally posted by @slint in #221 (comment)