title: Progressbar and error message with Paging Library
body: <p><a href="https://developer.android.com/topic/libraries/architecture/paging.html" rel="noreferrer">Paging Library</a> is amazing. But I find that lacks this features:</p>
  
  <ul>
  <li>Dispatch a view when data is loading. Assuming I am extending <code>PageKeyedDataSource</code>: a view on top when <code>loadInitial</code> is called, a view on bottom of the list when <code>loadAfter</code> is called. Views should disappear when <code>callback</code> is invoked. </li>
  <li>Dispatch a view when there is an error.</li>
  <li>Swipe to refresh</li>
  </ul>
  
  <p>As this is not possible right now, does anyone know a way to do it using PagingLibrary? At least <strong>a way to use different views in the same list</strong>. </p>