title: Any way to get more details on GCM failure?
body: <p>I'm currently working on a push notification API that will work with several apps at once, handling notifications and reducing programming time for future apps. It's already partially working, as I'm able to register and receive notifications on Android devices. </p>
  
  <p>Eventually, one of our apps is gonna send broadcast notifications to registered users. But some tokens might be expired, which will lead to a GCM failure. I already tested, and it seems that sending an array of tokens to GCM with a single http call is working really well, as devices with valid tokens got their notifications. </p>
  
  <p>What I wasn't able to find searching GCM documentation was a way to get more details in case of failure. For example, when I send a notification to two users, one with a valid token and the other with an invalid one, I got this result : </p>
  
  <pre><code>{
  "multicast_id":7625209716676388798,
  "success":1,
  "failure":1,
  "canonical_ids":0,
  "results":[
      {"error":"InvalidRegistration"},
      {"message_id":"0:1466511379030431%c4718df8f9fd7ecd"}
     ]
  }
  </code></pre>
  
  <p>We can see that one of the messages failed to send, but what I'm looking for is a way to get more details, ideally the token that leads to a failure, so I can remove it from my database. </p>
  
  <p>Any way to achieve that ? Using the message_id maybe ? Or is there any solution for me to find invalid tokens stored in my database so I can clear them ? I might have missed something in the documentation, even a link to it would be useful. </p>