The error handling in JsonResponse should not silent all errors

To make debugging easier, the error handling in https://github.com/symfony/symfony/blob/e797659fb5727df97d87b7c4d2eecb309cc83ad8/src/Symfony/Component/HttpFoundation/JsonResponse.php#L98 should not silent all errors but only the warnings coming from json_encode itself.

See https://github.com/symfony/symfony/commit/cf71e226b83b2aa332d5bd4effdc2375296b44fb#commitcomment-7045622 for more details.
