built-in server: exit when docroot does not exist

When the server:run command is run with an invalid document root
directory (for example, when being in the app directory and not
changing the document root to ../web/), the command crashes on Windows
with a 267 exit code. On Linux, the server starts but just publishes
internal server errors.

src/Symfony/Bundle/FrameworkBundle/Command/ServerRunCommand.php

@@ -81,6 +81,14 @@ protected function configure()
      */
     protected function execute(InputInterface $input, OutputInterface $output)
     {
+        $documentRoot = $input->getOption('docroot');
+
+        if (!is_dir($documentRoot)) {
+            $output->writeln(sprintf('<error>The given document root directory "%s" does not exist</error>', $documentRoot));
+
+            return 1;
+        }
+
         $env = $this->getContainer()->getParameter('kernel.environment');
 
         if ('prod' === $env) {
@@ -96,7 +104,7 @@ protected function execute(InputInterface $input, OutputInterface $output)
         $output->writeln(sprintf("Server running on <info>http://%s</info>\n", $input->getArgument('address')));
 
         $builder = new ProcessBuilder(array(PHP_BINARY, '-S', $input->getArgument('address'), $router));
-        $builder->setWorkingDirectory($input->getOption('docroot'));
+        $builder->setWorkingDirectory($documentRoot);
         $builder->setTimeout(null);
         $builder->getProcess()->run(function ($type, $buffer) use ($output) {
             if (OutputInterface::VERBOSITY_VERBOSE <= $output->getVerbosity()) {