Show maintenance error on 500 errors

[luistoptal]

Pull request for issue: #1341

Shows custom error message for 500 errors

How to test?

You can throw a 500 error from any controller

eg. grep $this->render('about'); then add the following before the render call

throw new CHttpException(500,'Mock Internal Server Error');

And navigate to http://gigadb.gigasciencejournal.com/site/about to confirm

How have functionalities been implemented?

Updated the protected/views/site/error.php with a custom error for 500 code