Improve messaging for JSON validation errors

[effigies]

trafficstars

• As in #90, do not all-caps the subcode
• This file is not a sidecar.
• The input is GeneratedBy: [{}]. It would be helpful if we could indicate that the error is in GeneratedBy[0].
• We could try to render the type, but perhaps it would make more sense to link to the BIDS glossary? https://bids-specification.readthedocs.io/en/stable/glossary.html#generatedby-metadata