How do I get errors back from REST service using WebClient?

I have a service that I call from Silverlight.  It is both WCF and REST. My Silverlight client calls it fine, however, when it is a WebClient call, an exception in the service does not come back to the client. I only get "Server not found", not the error message. For the WCF call, I use a SilverlightFaultBehavior on the server and that DOES get the error message of the exception back.

But either that SilverlightFaultBehavior is goofing up the REST exception, or I need to do something different.

Here is the REST call and completed event handler (in Oxygene language):

   function ViewModel.GetRESTVSDSInfo : VSDS.Data.DataContracts.VSDSInfo;
      begin
      IsBusy := true;

      VSDSInfo := nil;

      var wc := new WebClient;

      if UseJson
         then wc.Headers[HttpRequestHeader.Accept] := "application/json"
         else wc.Headers[HttpRequestHeader.Accept] := "application/xml";

      var uri := new Uri( Application.Current.Host.Source, '../../Service.svc/Rest/VSDSInfo' );

      wc.DownloadStringCompleted += wcRestVSDSInfoDownloadStringCompleted;
      wc.DownloadStringAsync( uri );
      end;

   method ViewModel.wcRestVSDSInfoDownloadStringCompleted(sender: Object; e: DownloadStringCompletedEventArgs);
      begin
      IsBusy := false;

      var wc := WebClient( sender );
      wc.DownloadStringCompleted -= wcRestVSDSInfoDownloadStringCompleted;

      if e.Error <> nil then begin
         MessageBox.Show( 'Error: ' + e.Error.Message  );
         exit;
         end;

      if UseJson
         then self.VSDSInfo := SerializationHelper.FromJson<VSDS.Data.DataContracts.VSDSInfo>( e.Result )
         else self.VSDSInfo := SerializationHelper.FromXml<VSDS.Data.DataContracts.VSDSInfo>( e.Result );

      SurveyHeader.VSDSInfo := self.VSDSInfo;
      InitData;
      end;

Here is the WCF call and completion handler:

   function ViewModel.GetWCFVSDSInfo : VSDS.Data.DataContracts.VSDSInfo;
      begin

      IsBusy := true;

      VSDSInfo          := nil;

      Proxy := ServiceProxy.NewClient;
      Proxy.GetVSDSInfoCompleted += ProxyGetWCFVSDSInfoCompleted;
      Proxy.GetVSDSInfoAsync;
      end;

   method ViewModel.ProxyGetWCFVSDSInfoCompleted(sender: Object; e: GetVSDSInfoCompletedEventArgs );
      begin
      IsBusy := False;
      if sender <> nil then begin
         Proxy.GetVSDSInfoCompleted -= ProxyGetWCFVSDSInfoCompleted;
         Proxy.CloseAsync;
         end;

      if e.Error <> nil then begin
         MessageBox.Show( 'Error: ' + e.Error.Message  );
         exit;
         end;

      self.VSDSInfo         := e.Result;
      SurveyHeader.VSDSInfo := self.VSDSInfo;
      InitData;
      end;

And here is the service code:

   function  VSDSService.GetVSDSInfo: VSDSInfo;
      begin
      try
         using db := new Database( 'VSDS' ) do begin
            result := db.GetVSDSInfo;
            end;

      except
         on e: Exception do begin
            raise new System.Data.Services.DataserviceException( e.Message );
            end;
         end;
      end;

I've tried a few different exception types.  Just a plain Exception, a WebFaultException, etc.  Nothing made a difference.

Here is the relevant web.config section for the SilverlightFaultBehavior:

<extensions><behaviorExtensions><add name="silverlightFaults" type="Silverlight.ServiceFaults.SilverlightFaultBehavior, SilverlightFaultBehavior"/></behaviorExtensions></extensions><behaviors><endpointBehaviors><behavior name="SilverlightFaultBehavior"><silverlightFaults/></behavior><behavior name="VSDSRestfulBehavior"><webHttp helpEnabled="true" automaticFormatSelectionEnabled="true"/><dataContractSerializer maxItemsInObjectGraph="2147483647"/></behavior></endpointBehaviors><serviceBehaviors><behavior name="VSDSBehavior"><serviceDebug includeExceptionDetailInFaults="true"/><serviceMetadata httpGetEnabled="true"/><!--<serviceMetadata httpsGetEnabled="true"/>--><dataContractSerializer maxItemsInObjectGraph="2147483647"/></behavior></serviceBehaviors></behaviors>