Aligned "no such task" error message with verbose messages.

The message now also starts with "TODO:", like all other messages (except for the usage messages).
Before: 42: No such task.
After:  TODO: No task 42.

tests/t1700-depri.sh

@@ -12,7 +12,7 @@ EOF
 
 test_todo_session 'depriority nonexistant item' <<EOF
 >>> todo.sh depri 42
-42: No such task.
+TODO: No task 42.
 === 1
 EOF