title: Is there a packaged_task::set_exception equivalent?
body: <p>My assumption is that <code>packaged_task</code> has a <code>promise</code> underneath. If my task throws an exception, how do I route that to the associated <code>future</code>? With just a <code>promise</code> I could call <code>set_exception
  </code> – how do I do the same for <code>packaged_task</code>?</p>