fix moderated grading speedgrader issue
fix moderated grading speedgrader issue

Fixes a bug where an error message would be shown after grading a
student as a provisional grader in SpeedGrader with the
'Edit Submission Status from Speedgrader' flag enabled.

closes EVAL-2161
flag=edit_submission_status_from_speedgrader

Test Plan:
1. Enable 'Edit Submission Status from Speedgrader'
2. Create a moderated assignment
3. In SpeedGrader, as a provisional grader, award a grade to a student
4. Notice that an error message is not shown. Refresh the page and
   verify the awarded grade is still there.

Change-Id: Ia2d58e1973af8a1a3315b95d0f706c1e39150dfc
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/281792
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Syed Hussain <1379c89cd1d253d3c5a299cc4d8f6002ada9d6b5@instructure.com>
Reviewed-by: Aaron Shafovaloff <d0ee6b99167c663aba7bc3efd806fe8d6cad437b@instructure.com>
QA-Review: Dustin Cowles <25afa0fec7fcf03720748195b533ef56026fd0cf@instructure.com>
Product-Review: Syed Hussain <1379c89cd1d253d3c5a299cc4d8f6002ada9d6b5@instructure.com>

sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms