give good messaging when a quiz question is big

fixes CNVS-6983

When submitting a quiz question that's too long,
the response that is "unknown error" which isn't
helpful.  This provides a useful error message.

test plan:
 - Login as a teacher
 - EDIT AN EXISTING quiz question
 - make the question text more than 16384 characters
 - save the question
 - you should get an error that tells you why it can't save
 - CREATE A NEW quiz question
 - make the question text more than 16384 characters
 - save the question
 - you should get an error that tells you why it can't save

Change-Id: Ida3438a9648c6b8c61f9e4a19c9414f3595a67e8
Reviewed-on: https://gerrit.instructure.com/39748
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>