Description of the enhancement:
Improve error message where you are unable to give a list item a child, if it has a child item in a different list
You are unable to give a list item a child, if it has a child item in a different list.
Error:
"Parent item has a child item that is in a different list: You can only add a child to a parent item, if any existing children of that parent item belong to the same list."
Can the error message be improved to say which lists and which items?
