improve handling router script paths

The `server:run` command switches the working directory before
starting the built-in web server. Therefore, the path to a custom
router script had to be specified based on the document root path
and not based on the user's working directory.

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

@@ -101,6 +101,14 @@ EOF
             ->locateResource(sprintf('@FrameworkBundle/Resources/config/router_%s.php', $env))
         ;
 
+        if (!file_exists($router)) {
+            $output->writeln(sprintf('<error>The given router script "%s" does not exist</error>', $router));
+
+            return 1;
+        }
+
+        $router = realpath($router);
+
         $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));