Fix explanation, if state change is blocked by contained checkboxes
* lisp/org.el (org-todo): Fix explanation, if state change is blocked
by contained checkboxes.
Consider a node, which contains unchecked checkboxes; if you have set
org-enforce-todo-checkbox-dependencies and try to change the node to
DONE, you will be denied with a message explaining why. However in this
special case the explanation would be wrong in talking of an unrelated
node instead of the checkboxes.
The fix uses the already existing variable org-blocked-by-checkboxes
(which is handled in org-block-todo-from-checkboxes). Similar code is
already present in org-agenda-dim-blocked-tasks within org-agenda.el.