Changes time of assignment update to after change

Fixes: SIS-645

assignment update happens after change made in
post grade dialog instead of after error count = 0

Test-Plan:
--Ensure at least 2 assignment have post grades error--

1.) Navigate to grade book and post grades
2.) Resolve error for one assignment
3.) Close modal and verify that assignment has been
	updated

Change-Id: I8cfa415631427e711fdbf1bec167d5376fecc8be
Reviewed-on: https://gerrit.instructure.com/44225
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Duane Johnson <duane@instructure.com>
Reviewed-by: Ken Romney <kromney@instructure.com>
Reviewed-by: Nick Houle <nhoule@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>