Use die() / print to stderr for error conditions

To indicate that something went wrong (e.g. the task already was unprioritized).
Note: For actions that handle multiple ITEMs in a loop, we cannot use die() as that would abort processing of any following ITEM(s). Instead, use a status variable and set it to 1 on error, then exit at the end.

tests/t1700-depri.sh

@@ -82,6 +82,7 @@ test_todo_session 'depriority of unprioritized task' <<EOF
 TODO: 3 of 3 tasks shown
 
 >>> todo.sh depri 3 2
+=== 1
 TODO: 3 is not prioritized.
 2 notice the sunflowers
 TODO: 2 deprioritized.