Better error message when the root node can't be deleted because it has children.
Better error message when the root node can't be deleted because it has children.

htem/CATMAID,fzadow/CATMAID,fzadow/CATMAID,htem/CATMAID,fzadow/CATMAID,htem/CATMAID,fzadow/CATMAID,htem/CATMAID