Error for invalid filter is now auto hidden on a new filter submission.

static/quiz_item.coffee

@@ -82,6 +82,7 @@ filterQuestions = () ->
 
 $('#filter-form').submit( (e) ->
   e.preventDefault()
+  $('#error-area').hide()
   filterQuestions()
   $('#modal-filter').modal('hide')
   linearCount = 0

static/quiz_item.js

@@ -127,6 +127,7 @@
 
   $('#filter-form').submit(function(e) {
     e.preventDefault();
+    $('#error-area').hide();
     filterQuestions();
     $('#modal-filter').modal('hide');
     return linearCount = 0;