title: Sidekiq Pro callback when batch is retried?
body: <p>I'm using sidekiq pro for my application, and it's been working great. But I'd like to have a way to notify my users that a failed job is being retried. </p>
  
  <p>A flow would go something like this:</p>
  
  <p><code>Batch starts
  worker1 runs successfully
  worker2 runs successfully
  worker3 fails
  oncomplete fires, stuff happens
  worker3 restarts
  ** onretry fires, notification sent to user
  worker 3 runs successfully
  onsuccess fires, stuff happens</code></p>
  
  <p>My imaginary onretry doesn't exist in the documentation but I'm hoping there's a way to fake it. I know that I can tell if the batch has failures via the status object, but I don't see a way to get a retry event. Is there such a thing?</p>