Add HTTP 400 Bad Request as a possible generic error response

[arttuperala]

Description of the Change

Adds 400 Bad Request as a possible error response, as JSON:API specification says to use it for requests with bad include / sort or otherwise illegal query parameters, and advocates using it as an appropriate status code for multiple 4xx errors. DJA's documentation also mentions 400 Bad Request multiple times, so it's a bit weird if it's not documented as a possible response.

Discovered during an OpenAPI schema validation run as our generic error responses were not found in the schema.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[sliverc]

Thanks. Indeed, that makes sense. Merging.