[DX] Improved error message on wrong error

Someone on IRC got `Warning: constant(): Couldn't find constant \FOS\RestBundle\Util\Codes:: in /../vendor/friendsofsymfony/rest-bundle/FOS/RestBundle/DependencyInjection/FOSRestExtension.php line 137` when configuring an empty code. This will make the error a bit more debuggable and nicer :)