Fixed bug when user votes w/o permission - now displays a friendly error message instead of failing.

Fixed bug when hinter module displays a hint, then is asked to display nothing.  (Used to not update in this case.)

common/lib/xmodule/xmodule/crowdsource_hinter.py

@@ -227,7 +227,7 @@ class CrowdsourceHinterModule(CrowdsourceHinterFields, XModule):
         Returns key 'hint_and_votes', a list of (hint_text, #votes) pairs.
         """
         if self.user_voted:
-            return json.dumps({'contents': 'Sorry, but you have already voted!'})
+            return {}
         ans_no = int(get['answer'])
         hint_no = str(get['hint'])
         answer = self.previous_answers[ans_no][0]

common/lib/xmodule/xmodule/js/src/crowdsource_hinter/display.coffee

@@ -72,3 +72,5 @@ class @Hinter
       JavascriptLoader.executeModuleScripts @el, () =>
         @bind()
       @$('#previous-answer-0').css('display', 'inline')
+    else
+      @el.hide()

common/templates/hinter_display.html

@@ -95,13 +95,17 @@ What would you say to help someone who got this wrong answer?
 </%def>
 
 <%def name="show_votes()">
-    Thank you for voting!
-    <br />
-    % for hint, votes in hint_and_votes:
-        <span style="color:green"> ${votes} votes. </span>
-        ${hint}
-        <br />
-    % endfor
+    % if hint_and_votes is UNDEFINED:
+      Sorry, but you've already voted!
+    % else:
+      Thank you for voting!
+      <br />
+      % for hint, votes in hint_and_votes:
+          <span style="color:green"> ${votes} votes. </span>
+          ${hint}
+          <br />
+      % endfor
+    % endif
 </%def>
 
 <%def name="simple_message()">
@@ -123,3 +127,4 @@ What would you say to help someone who got this wrong answer?
 % if op == "vote":
     ${show_votes()}
 % endif
+