If the error happens during DELETE IGNORE, nothing could be send to the
client, thus leaving it frozen expecting the reply.

The problem was that if some error occurred, it wouldn't be reported to
the client because of IGNORE, but neither success would be reported.

MySQL 4.1 would not freeze the client, but will report

  ERROR 1105 (HY000): Unknown error

instead, which is also a bug.

The solution is to report success if we are in DELETE IGNORE and some
non-fatal error has happened.