如何在RESTful API中为POST方法设置正确的JSON响应

我正在为RESTful API制作POST方法。正如你可能注意到的，这个API是建立在FOSRestBundle和NelmioApiDoc之上的。当文件未上传或缺少rid参数并使用适当的JSON响应时，我无法验证。这就是我正在做的:

/**
 * Set and upload avatar for reps.
 *
 * @param ParamFetcher $paramFetcher
 * @param Request $request
 *
 * @ApiDoc(
 *      resource = true,
 *      https = true,
 *      description = "Set and upload avatar for reps.",
 *      statusCodes = {
 *          200 = "Returned when successful",
 *          400 = "Returned when errors"
 *      }
 * )
 *
 * @RequestParam(name="rid", nullable=false, requirements="'d+", description="The ID of the representative")
 * @RequestParam(name="avatar", nullable=false, description="The avatar file")
 *
 * @return View
 */
public function postRepsAvatarAction(ParamFetcher $paramFetcher, Request $request)
{
    $view = View::create();
    $uploadedFile = $request->files;
    // this is not working I never get that error if I not upload any file
    if (empty($uploadedFile)) {
        $view->setData(array('error' => 'invalid or missing parameter'))->setStatusCode(400);
        return $view;
    }
    $em = $this->getDoctrine()->getManager();
    $entReps = $em->getRepository('PDOneBundle:Representative')->find($paramFetcher->get('rid'));
    if (!$entReps) {
        $view->setData(array('error' => 'object not found'))->setStatusCode(400);
        return $view;
    }
    .... some code
    $repsData = [];
    $view->setData($repsData)->setStatusCode(200);
    return $view;
}

如果我没有上传文件，我得到这样的响应:

Error: Call to a member function move() on a non-object
500 Internal Server Error - FatalErrorException

但是作为Symfony异常错误不像我想要和需要的JSON，所以代码永远不会进入if。

如果我没有设置rid，那么我得到了这个错误:

Request parameter "rid" is empty
400 Bad Request - BadRequestHttpException

但再次作为Symfony异常错误，而不是作为JSON。如果rid不存在或文件未上传，我如何响应适当的JSON ?任何建议吗?