Send a new event from interface.js whenever a helpful message is show…

…n after user attempts a problem (e.g. 'It looks like you haven't answered the entire problem yet').

Summary: Event will be used by mobile.

Test Plan:
Drop into mobile repo w/ my prototype that listens for this event
Verify that event is fired and sends appropriate message along.

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D10466

interface.js

@@ -219,12 +219,18 @@ function handleAttempt(data) {
     }
 
     var isAnswerEmpty = score.empty && !skipped;
+    var attemptMessage = null;
 
-    // Is this a message to be shown?
+    // Is there a message to be shown?
     if (score.message != null) {
-        $("#check-answer-results > p").html(score.message).show().tex();
+        attemptMessage = score.message;
     } else if (isAnswerEmpty) {
-        $("#check-answer-results > p").html(EMPTY_MESSAGE).show().tex();
+        attemptMessage = EMPTY_MESSAGE;
+    }
+
+    if (attemptMessage) {
+        $("#check-answer-results > p").html(attemptMessage).show().tex();
+        $(Exercises).trigger("attemptMessageShown", attemptMessage);
     } else {
         $("#check-answer-results > p").hide();
     }

khan-exercise.js

@@ -53,6 +53,9 @@
 
     * showGuess -- when a guess is populated in the answer area in problem
       history mode
+
+    * attemptMessageShown -- when a user attempts a problem and a message is
+      shown in response, e.g. "We don't understand your answer."
 */
 define(function(require) {