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/t1910-deduplicate.sh

@@ -32,6 +32,7 @@ EOF
 
 test_todo_session 'deduplicate without duplicates' <<EOF
 >>> todo.sh deduplicate
+=== 1
 TODO: No duplicate tasks found
 EOF