Quiz editing: errors in Ajax calls are not reported

Steps to reproduce (note, these steps depend on MDL-57228 not being fixed):

1. Create a quiz like
   • Section heading 1
   • Question 1
   • Section heading 2
   • Question 2
   • Section heading 3
   • Question 3
   • Question 4
2. Try to drag Question 4 into Section 1.

Actual result: it appears to work, but if you reload the page, Question 4 goes back to where it was. If you open developer tools Network tab, you can see the Ajax call gave an error.

Expected result: if an error occurs with the Ajax call, then it is reported to the user, and the page is not changed. (Or, when the error dialogue is dismissed, the page is forced to reload.)