REST API: Improve JSON Schema validation error codes

[TimothyBlynJacobs]

rest_validate_value_from_schema currently has a mix of error codes in use. Some are nice and specific like rest_invalid_date but most are the unhelpful rest_invalid_param. Right now, this doesn't have much of an impact since the individual errors from rest_validate_value_from_schema are not returned to clients. But in preparation for #46191, we should take this opportunity to make them more useful, as after it may be more of a BC break.

Trac ticket: https://core.trac.wordpress.org/ticket/52317

[TimothyBlynJacobs]

In 50007:

REST API: Add more specific error codes for schema validation.

Previously, the majority of JSON Schema validation errors returned a generic rest_invalid_param error code. In preparation for #46191, where the underlying validation error code will be exposed, this commit adds specific error codes for each failure scenario.

Fixes #52317.

Merged in ​https://github.com/WordPress/wordpress-develop/commit/15c8166424a2f20419d0397783867f276e178ce5.

[TimothyBlynJacobs]

Reopening to backport r50461 to 5.7.

[desrosj]

[50461] looks good to backport.

[SergeyBiryukov]

In 50470:

REST API: Correct error code for the maxItems keyword.

Follow-up to [50007].

Props TimothyBlynJacobs.
Reviewed by desrosj, SergeyBiryukov.
Merges [50461] to the 5.7 branch.
Fixes #52317.