[ServiceWorker] Not Found Error instead of undefined response (#666)

I feel it's a little bit weird when `Not Found` caches return `undefined` response instead of a `Not Found` error, because common IO operations often solve this situation with an error.
```
caches.match('/app.js').then(function(response){
     console.log("Resource found");
}).catch(function(er){
     if(er.code === 'NOT_FOUND'){
          console.log("Resource Not Found");
     }
});

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/666