fixes points possible on letter grade

fixes the ability to add 00 for points possible.
You may be able to enter in 0 and 00 but an
error will show.

fixes CNVS-16397

test plan:
- create a new assignment
- set the 'display grade as' option to letter grade
- give a points possible and save the assignment.
- go back and change the points possible to something
  other than what it was. Notice error message.
- try to save the assignment with points less
  than 0 notice error.

Change-Id: Id8106ab40425f6f48de2eab3ac5aa730c8197512
Reviewed-on: https://gerrit.instructure.com/45451
Product-Review: Hilary Scharton <hilary@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Adam Stone <astone@instructure.com>
Reviewed-by: Matt Berns <mberns@instructure.com>