Multiple errors via param validation should be programmatically readable #1143

Currently if a request contains multiple invalid request args, we compile an error string showing all the issues. However, we don't pass all the different fields that errored. From @rmccue in slack:

new WP_Error( 'json_missing_field', 'Missing $field', array( 'key' => $field ) )
Would output as [ { "code": "json_missing_field", "message": "Missing abc", "data": { "key": "abc" } }]