drupal_site_offline() has been removed

The drupal_site_offline() function has been removed. In previous Drupal versions it allowed modules to arbitrarily display a message that the site is in maintenance mode, even if it wasn't. That is not accurate. Instead, code that fails badly enough that it cannot recover should throw a meaningful exception with an appropriate error message, preferably of a meaningful type rather than simply the Exception class. If the exception is not otherwise handled, the system will display a formatted page with an HTTP 500 error message and the supplied error message.