Add proper error_notification_class for Bootstrap 3

Commit a685ba45e06276ad90a4000cf12d82b333f7e6f4 rewrites some classes, but completely removes the config.error_notification_class in the Bootstrap override. Because of this, error notifications no longer show up as an alert.

I've added the override, using the correct Bootstrap 3 alert class `alert-danger`, which is the proper replacement voor `alert-error`.