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/t1500-do.sh

@@ -81,6 +81,7 @@ test_todo_session 'fail multiple do attempts' <<EOF
 TODO: 3 marked as done.
 
 >>> todo.sh -a do 3
+=== 1
 TODO: 3 is already marked done.
 EOF