add ended to the worksheet theme's overriden score feedback object

Not sure why this was only a problem on the mathcentre editor, not in my dev environment.

Author: christianp

themes/worksheet/files/scripts/worksheet-display.js

@@ -247,6 +247,7 @@ display_util.showScoreFeedback = function(obj,settings)
     const obj2 = {
         answered: () => false,
         isDirty: () => false,
+        ended: () => false,
         score: () => 0,
         marks: () => Knockout.unwrap(obj.marks),
         credit: () => 0,