not know there's rollback (if it's because of a dupl row), better warn
that it's happening. It can also be of use for a DBA killing a
connection and wondering what this connection is still doing now. Example:
|  5 | root | localhost | test | Killed  | 10   | Rolling back | insert into i select * from j |


sql/handler.cc:
  As rollback can be 30 times slower than insert in InnoDB, and user may
  not know there's rollback (if it's because of a dupl row), better warn.