Replace _dialogCoordinator.ShowMessageAsync in Export() task

[BornToBeRoot]

Replace this part with a childWindow message.

 catch (Exception ex)
 {
     Log.Error("Error while exporting data as " + instance.FileType, ex);

     var settings = AppearanceManager.MetroDialog;
     settings.AffirmativeButtonText = Strings.OK;

     await _dialogCoordinator.ShowMessageAsync(this, Strings.Error,
         Strings.AnErrorOccurredWhileExportingTheData + Environment.NewLine +
         Environment.NewLine + ex.Message, MessageDialogStyle.Affirmative, settings);
 }