exit when Twig environment is not set

The `LintCommand` from the TwigBridge requires the `Twig_Environment`
to be set. Thus, if the `setTwigEnvironment()` were not called, the
command execution would have been aborted with a PHP error.

src/Symfony/Bridge/Twig/Command/LintCommand.php

@@ -87,6 +87,13 @@ EOF
     protected function execute(InputInterface $input, OutputInterface $output)
     {
         $twig = $this->getTwigEnvironment();
+
+        if (null === $twig) {
+            $output->writeln('<error>The Twig environment needs to be set.</error>');
+
+            return 1;
+        }
+
         $filename = $input->getArgument('filename');
 
         if (!$filename) {