what is the event by which i can get that user deny the offline data store when the message comes in the browser

I have an application which stores manifest for offline use. Now firefox asking for a confirm message that if i want to allow or not. If i say 'allow' the application works fine. but if i say 'not now' or 'never for this site' the application stops. What is the event to check that user deny to allowing store data. i used different option like - cached,noupdate,error,uncached,idle,obsolete. as an example- applicationCache.addEventListener('idle', function () {});

but none of this says that user deny to allow. Please tell me the option that i can check that user does not want to allow and do my rest work.