Fix old rubric criterion edit error message

fixes EVAL-4864
flag=enhanced_rubrics

test plan:
- disable FF ehanced_rubrics
- create or edit a rubric
- add a criterion or a rating
- validate the description or rating title
has the rigth error message when save button
is clicked and the field is empty

Change-Id: I88dabf88af30d632b592728e7cb24aef440e5ea0
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/369955


Reviewed-by: Martin Dubai <m.dubai@instructure.com>
QA-Review: Martin Dubai <m.dubai@instructure.com>
Product-Review: Martin Dubai <m.dubai@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>