$action = Yii::app()->getController()->getAction()->getId() or $this->getAction()->getId()
$id = Yii::app()->getRequest()->getQuery('id');
throw new CHttpException(404,Yii::t('yii','The system is unable to find the requested action "{$action}/{$id}");
ok, comments the actionNopage method in your Site Controller. the site/signin will be routed to site/nopage tha is not exists. So errorHandler action catch the exception (actionError() in site Controller) and automatically displays which of controller/action is not exist.