Extend error handling for ability schema validation

[gziolo]

Follow-up for:

• Add REST API controllers for Abilities API #6

REST API needs detailed error messages with HTTP status codes. While WP_Ability::execute() validates internally, it only returns false and logs with _doing_it_wrong(), which doesn't provide capturable error messages. Related code that handles it today:

https://github.com/WordPress/abilities-api/blob/trunk/includes/rest-api/endpoints/class-wp-rest-abilities-run-controller.php#L137L144

https://github.com/WordPress/abilities-api/blob/dd94fce1a6ba8930d39f2011fa1e6147f9759ad8/includes/rest-api/endpoints/class-wp-rest-abilities-run-controller.php#L164L169

Consider updating WP_Ability to return WP_Error for better error handling which would then simplify and unify the implementation in REST API controller.

[justlevine]

@gziolo I believe #23 should close this.

[gziolo]

After landing #23, there is still some follow-up work needed to improve REST API integration with ability execution.

[gziolo]

I work on a follow-up in #31.