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/t1200-pri.sh

@@ -90,6 +90,7 @@ TODO: 2 re-prioritized from (C) to (A).
 TODO: 3 of 3 tasks shown
 
 >>> todo.sh pri 2 a
+=== 1
 2 (A) notice the sunflowers
 TODO: 2 already prioritized (A).