JSON API 403 errors don't tell the user *why* access is not granted: requires deep Drupal understanding to figure out

Problem/Motivation

#2808233: REST 403 responses don't tell the user *why* access is not granted: requires deep Drupal understanding to figure out fixed this for Drupal core: it introduced the necessary infrastructure and made the REST module use it. #2824572: Write EntityResourceTestBase subclasses for every other entity type. then improved the "403 reason" for many entity types.

Let's give JSON API users the same vastly improved DX.

Proposed resolution

Update \Drupal\jsonapi\Controller\EntityResource::getIndividual() and friends.

Remaining tasks

TBD

User interface changes

None.

API changes

More helpful error messages than 'The current user is not allowed to POST the selected resource.'.

Data model changes

None.