Add the missing group reference identification to exception.

lib/timeline/component/LineGraph.js

@@ -464,7 +464,7 @@ LineGraph.prototype._updateAllGroupData = function () {
       if (this.itemsData._data.hasOwnProperty(itemId)) {
         var item = this.itemsData._data[itemId];
         if (groupsContent[item.group] === undefined) {
-          throw new Error('Cannot find referenced group. Possible reason: items added before groups? Groups need to be added before items, as items refer to groups.')
+          throw new Error('Cannot find referenced group ' + item.group + '. Possible reason: items added before groups? Groups need to be added before items, as items refer to groups.')
         }
         item.x = util.convert(item.x,'Date');
         groupsContent[item.group].push(item);