server:run command: provide more error information

The server:run command didn't provide many information when the executed
command exited unexpectedly. Now, the process' exit code is passed through
and an error message is displayed.

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

@@ -102,6 +102,16 @@ EOF
         $this
             ->getHelper('process')
             ->run($output, $process, null, null, OutputInterface::VERBOSITY_VERBOSE);
+
+        if (!$process->isSuccessful()) {
+            $output->writeln('<error>Built-in server terminated unexpectedly</error>');
+
+            if ($process->isOutputDisabled()) {
+                $output->writeln('<error>Run the command again with -v option for more details</error>');
+            }
+        }
+
+        return $process->getExitCode();
     }
 
     private function createPhpProcessBuilder(InputInterface $input, OutputInterface $output, $env)