fix discussion entry form quota error handling
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 <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
Tested-by: Jenkins
QA-Review: Jahnavi Yetukuri <ed2b2672ccc2e59e56c0615df7f01b579202e014@instructure.com>
Product-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

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