Kanban view doesn't show constraint error message

Problem description:
When dragging an item from one stage to another in the kanban view, it's possible to get the following error message:
"An error has occured while moving the record to this group: undefined"

It can for example happen when a constraint of the item is no longer valid (constraint depends on some factor elsewhere in OpenERP). When moving the item, the constraint is validated (since the item is changed), which blocks the action and results in the error.

Relevant code from kanban.js (web_kanban module):
            var data = {};
            data[this.group_by] = new_group.value;
            this.dataset.write(record.id, data, {}).done(function() {
                record.do_reload();
                new_group.do_save_sequences();
            }).fail(function(error, evt) {
                evt.preventDefault();
                alert(_t("An error has occured while moving the record to this group: ") + data.fault_code);
                self.do_reload(); // TODO: use draggable + sortable in order to cancel the dragging when the rcp fails
            });

In the alert, 'data.fault_code' is used. The variable doesn't have an attribute fault_code (resulting in 'undefined'). However, the 'error' variable has a variable 'data', which does have a variable 'fault_code'. So instead of 'data.fault_code', 'error.data.fault_code' can be used.