Validation exceptions on launcher HTTP response are double JSON encoded

Problem/Motivation

\Drupal\Core\EventSubscriber\ExceptionJsonSubscriber takes the exception message and wraps the exception message into an array and encodes it. This means we're currently getting a weird double encoding when returning an array of constraint errors

      $messages = array_map(static function (ConstraintViolationInterface $violation) {
        return sprintf("%s: %s", $violation->getPropertyPath(), $violation->getMessage());
      }, \iterator_to_array($constraints));
      throw new UnprocessableEntityHttpException(Json::encode([
        'errors' => $messages,
      ]));

There isn't a good way to return an array of errors. We'd have to borrow something like \Drupal\jsonapi\EventSubscriber\DefaultExceptionSubscriber but for the regular JSON format.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes