fix failing spec on quiz

fixed:
- since we're not explicitly checking for errors in quiz_submission#save, throw
  an exception if an error happens.
- fix error formatting assertion

Change-Id: I1ac2134a965d329e733c8f54ed2786e06adff91e
Reviewed-on: https://gerrit.instructure.com/29703
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>
QA-Review: Derek DeVries <ddevries@instructure.com>