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/t1600-append.sh

@@ -41,7 +41,7 @@ EOF
 test_todo_session 'append error' << EOF
 >>> todo.sh append 10 "hej!"
 === 1
-10: No such task.
+TODO: No task 10.
 EOF
 
 cat > todo.txt <<EOF