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>

app/models/quizzes/quiz.rb

@@ -309,7 +309,7 @@ class Quizzes::Quiz < ActiveRecord::Base
     # If the quiz suddenly changes from non-graded to graded,
     # then this will update the existing submissions to reflect quiz
     # scores in the gradebook.
-    self.quiz_submissions.each { |s| s.save }
+    self.quiz_submissions.each { |s| s.save! }
   end
 
   attr_accessor :saved_by

spec/models/quizzes/quiz_spec.rb

@@ -972,7 +972,7 @@ describe Quizzes::Quiz do
       it "should not allow quiz points higher than allowable by postgres" do
         q = Quizzes::Quiz.new(:points_possible => 2000000001)
         q.valid?.should == false
-        q.errors.on(:points_possible).should == "must be less than or equal to 2000000000"
+        Array(q.errors[:points_possible]).should == ["must be less than or equal to 2000000000"]
       end
     end