fixed InstallCommand exit-code

due to the symfony command line interface implementation it is required to set the autoExit to false in order to run multiple commands
by setting autoExit to true for this case symfony cli will now exit with exit-code 1

fixes #533

src/N98/Magento/Command/Installer/InstallCommand.php

@@ -681,7 +681,8 @@ protected function installMagento(InputInterface $input, OutputInterface $output
         exec($installCommand, $installationOutput, $returnStatus);
         $installationOutput = implode(PHP_EOL, $installationOutput);
         if ($returnStatus !== self::EXEC_STATUS_OK) {
-            throw new \Exception('Installation failed.' . $installationOutput);
+            $this->getApplication()->setAutoExit(true);
+            throw new \RuntimeException('Installation failed.' . $installationOutput, 1);
         } else {
             $output->writeln('<info>Successfully installed Magento</info>');
             $encryptionKey = trim(substr($installationOutput, strpos($installationOutput, ':') + 1));