In the grader report, with AJAX editing switched on, markers are given an editing interface which allows them to change the grade. However, when trying to save the grade, a message pops up saying "TO BE LOCALISED: Failure to update grade!"

To reproduce:

1. Manually create a grade item in a course
2. Lock the item
3. Make sure that AJAX editing is on for the grader report
4. Go into the grader report and click on the item for a student. You should see an editing interface (either a text box or a dropdown menu if using a scale)
5. Enter or select a grade and press return or move focus elsewhere on the page to save the grade
6. You see the error message "TO BE LOCALISED: Failure to update grade!"

The behaviour I would expect in this case would be that the editing interface is not presented, rather than having a more user-friendly error message such as "This grade cannot be edited as it is locked".

Just to be clear, this is not a duplicate of MDL-31658, although that issue appears to have been reported in response to a forum thread detailing the issue described here. (https://moodle.org/mod/forum/discuss.php?d=196114)