Corrected FatalErrorException namespace

`App::fatal` currently does nothing because it's listening for a `Symfony\Component\HttpKernel\Exception\FatalErrorException`, which has been [deprecated since 2.3](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpKernel/Exception/FatalErrorException.php) and replaced by `Symfony\Component\Debug\Exception\FatalErrorException`.