public static function process_booking_unauthorized_exceptions()
 {
     if (!self::$old_exception_handler) {
         self::$old_exception_handler = set_exception_handler(array(__CLASS__, 'handle_booking_unauthorized_exception'));
         if (!self::$old_exception_handler) {
             //The exception handler of phpgw has probably not been activated,
             //so taking that as a hint to not enable any of our own either.
             restore_exception_handler();
         }
     }
 }