Exception handler needs to handle exception subclassing

The exception handler currently only uses get_class() to determine the type of exception:

	/**
	 * Convert err to HttpException.
	 *
	 * @param mixed $err
	 *
	 * @return HttpException
	 */
	protected function toHttpException(Exception $err): HttpException
	{
		$errClass = get_class($err);

		switch ($errClass) {
			case HttpException::class:
				$httpErr = $err;
				break;

			// 404 Model not found.
			case ModelNotFoundException::class:
				$httpErr = $this->convertFromModelNotFound($err);
				break;

			case QueryException::class:
			default:
				$httpErr = $this->defaultHttpException();
				break;
		}

		return $httpErr;
	}

So throwing a BadRequestHttpException gets serialized to a 500 error instead of a 400 error.

Start at the toHttpException method shown in the issue and inspect how exception classes are currently selected. Verify the behavior for BadRequestHttpException and other HttpException subclasses, then confirm that a bad-request exception produces a 400 response rather than a 500 response.