title: webclient returns unknown error
body: <p>I have a simple scenario where my app makes an async web request for xml file (mock for webservice). </p>
  
  <p>Everything seems ok but webclient always returns UnknownError.</p>
  
  <p>What am I missing here?</p>
  
  <p>Here is the web request code:</p>
  
  <pre><code>public void LoadNews()
  {
      // to prevent caching append random string to uri
      Random rnd = new Random();
      Uri uri = new Uri(FeedUri +"?rnd="+rnd.Next());
  
      WebClient wc = new System.Net.WebClient();
  
  
      var o = Observable.FromEvent&lt;DownloadStringCompletedEventArgs&gt;(wc, "DownloadStringCompleted")
          .ObserveOn(Scheduler.ThreadPool)
          .Select(s =&gt; s.EventArgs.Result)
          .ObserveOn(Scheduler.Dispatcher)
          .Subscribe(r =&gt;
              {
              // use caliburn.micro eventaggregator to publish message
              EventAggregatorHelper.EventAggregator.Publish(new NewsPostMessage(LoadNewsPosts(r)));
              }, () =&gt; { });
  
      wc.DownloadStringAsync(uri);
  }
  </code></pre>
  
  <p>Here is the helper class:</p>
  
  <pre><code>public class EventAggregatorHelper
  {
      static EventAggregator _eventAggregator = new EventAggregator();
      public static EventAggregator EventAggregator
      {
          get { return _eventAggregator; }
      }
  }
  </code></pre>