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/t1800-del.sh

@@ -60,6 +60,7 @@ test_todo_session 'del with confirmation' <<EOF
 TODO: 3 of 3 tasks shown
 
 >>> printf n | todo.sh del 1
+=== 1
 Delete '(B) smell the uppercase Roses +flowers @outside'? (y/n)$SPACE
 TODO: No tasks were deleted.
 
@@ -71,10 +72,12 @@ TODO: No tasks were deleted.
 TODO: 3 of 3 tasks shown
 
 >>> printf x | todo.sh del 1
+=== 1
 Delete '(B) smell the uppercase Roses +flowers @outside'? (y/n)$SPACE
 TODO: No tasks were deleted.
 
 >>> echo | todo.sh del 1
+=== 1
 Delete '(B) smell the uppercase Roses +flowers @outside'? (y/n)$SPACE
 TODO: No tasks were deleted.