code: function (viewId, requirePath, err) {
              var that = this,
                  message = 'View Not Found. Searched for "' + viewId + '" via path "' + requirePath + '".';
  
              return system.defer(function(dfd) {
                  dfd.resolve(that.processMarkup('<div class="durandal-view-404">' + message + '</div>'));
              }).promise();
          }
docs: Called when a view cannot be found to provide the opportunity to locate or generate a fallback view. Mainly used to ease development.
  @method createFallbackView
  @param {string} viewId The view id whose view should be created.
  @param {string} requirePath The require path that was attempted.
  @param {Error} requirePath The error that was returned from the attempt to locate the default view.
  @return {Promise} A promise for the fallback view.