org.el: fix error message when C-u C-u C-c C-c on a list.

org.el: fix error message when C-u C-u C-c C-c on a list.

* org.el (org-ctrl-c-ctrl-c): Only throw a message when using
two universal prefix arguments on a list where all items are
already in a transitory state.  Refine the error when the
checkbox cannot be toggled.

Thanks to Robert Horn for reporting this.