DELETE IGNORE

The ER_CANT_UPDATE_USED_TABLE_IN_SF_OR_TRG error was set in the
diagnostics area when it happened, but the DELETE cleanup code
never checked for a non-fatal error condition, thus trying to
set diag.area to "ok".  This triggered an assert checking that
the diag.area was empty.

The fix was to test if there existed a non-fatal error condition
(thd->is_error() before ok'ing the operation.