The existing check didn't do much good, and one could deprioritize any normal task without notice. Instead, check that the task to be deprioritized actually has a priority and alert via "TODO: 42 is not prioritized." if that is not the case.
I.e. removed the colon after the ITEM#. Updated tasks are now listed like the 'todo.sh ls' command does, e.g.
42 smell the roses
This is in preparation to eventually use a function factored out from _list() for the verbose task output, which would enable coloring in the verbose message and thus align the verbose message output even more with the normal task output.
