fix discussion entry form quota error handling

test plan:
* create a student with their files over quota
(the easiest way to do this would probably be to edit
 the "User" quota setting for the account to a low number
 temporarily)
* create a course for the student, and enable
 "Let students attach files to discussion"
 (in More Options)
* create and publish a discussion topic

* as the student try to attach a file to a reply
 to the topic
* it should show an error (instead of pretending to
succeed and disappearing the entry)

closes #CNVS-27062

Change-Id: I33491bc390e429ec5e9bd8b9b7203ba8b874719a
Reviewed-on: https://gerrit.instructure.com/71630
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>

app/coffeescripts/discussions/Reply.coffee

@@ -10,6 +10,7 @@ define [
   'compiled/views/editor/KeyboardShortcuts'
   'str/stripTags'
   'tinymce.editor_box'
+  'jquery.instructure_forms'
 ], (Backbone, _, I18n, $, Entry, htmlEscape, replyAttachmentTemplate, preventDefault, KeyboardShortcuts, stripTags) ->
 
   class Reply
@@ -126,7 +127,13 @@ define [
     # Callback when the model is succesfully saved
     #
     # @api private
-    onPostReplySuccess: (entry) =>
+    onPostReplySuccess: (entry, response) =>
+      if response.errors
+        @hideNotification()
+        @textArea.val entry.get('message')
+        @edit()
+        @form.formErrors(response)
+      else
         @view.model.set 'notification', ''
         @trigger 'save', entry