title: How to access Cache.addAll() request array
body: <p>Having a simple service worker updating, when receiving a message, as below. This works fine and the cache is updated. But next is to leave one of the files inaccessible and trying to get some notification that one is failing. How to list results of the requests?</p>
  
  <p>Looking here
  [<a href="https://developer.mozilla.org/en-US/docs/Web/API/Cache/addAll][1]" rel="nofollow">https://developer.mozilla.org/en-US/docs/Web/API/Cache/addAll][1]</a></p>
  
  <p>States
  "The request objects created during retrieval become keys to the stored response operations."
   How is this interpretated? and in code accessed?</p>
  
  <pre><code>self.addEventListener('message',  event =&gt; {
    console.log('EventListener message ' + event.data);
  
   event.waitUntil(
      caches.open('minimal_sw').then(cache =&gt; {
         return cache.addAll(fileList).then(function(responseArray){
           console.log('EventListener responseArray ' + responseArray);
           self.skipWaiting();
         });
      })
    )
  });
  </code></pre>