title: How to know if a Worker has failed being triggered/executed by the queue?
body: <p>I am appending workers in the queue as shown below. However, occasionally a worker won't be triggered. How can I know if a Worker did not get executed so that I can try to re-enqueue it? Maybe using the <code>Operation</code> instance returned by <code>enqueue()</code>? How to use it? If so, is it possible to have a hint of the reason of failure of triggering the Worker?</p>
  
  <p>Note that I do not need to know if the Worker failed (<code>return Result.FAILURE</code>). The case I want to cover is when the Worker is not even called, which happens occasionally.</p>
  
  <pre><code>OneTimeWorkRequest sendMessageWorker = new OneTimeWorkRequest.Builder(SendMessageWorker.class)
          .setInputData(data)
          .setBackoffCriteria(BackoffPolicy.LINEAR, backOffDelayInMillis, TimeUnit.MILLISECONDS)
          .build();
  
  Operation operation = WorkManager.getInstance()
          .beginUniqueWork(SendMessageWorker.class.getSimpleName(), ExistingWorkPolicy.APPEND, sendMessageWorker)
          .enqueue();
  </code></pre>