More descriptive error messages in the quiz service.

Previously used to be along the lines of "Error saving quiz: [Object object]". Now, the error object is extracted to obtain the message and stack trace.

TO DO: It's not a good idea to be showing the end user a stack trace. However, as long as this is still in development, it makes life easier for me.
Edit Quiz: Make the quiz model be a property on the $scope, instead of using $scope itself as the model.  This also fixes the issue where changes to quiz name don't get auto-saved (since we're not watching the entire $scope.quiz model for changes, instead of just $scope.questions).
Fix new quiz creation
Strategic use of ng-cloak to improve loading of quiz editor
Edit Quiz: Trigger auto-save when reordering questions.
Edit Quiz: Show spinner while saving