mode

When the master was executing in sql_mode='traditional' (which
implies that really_abort_on_warning returns TRUE - because of
MODE_STRICT_ALL_TABLES), the error code (ER_DUP_ENTRY in the
reported case) was not being set in the
Query_log_event. Therefore, even if a failure was to be expected
when replaying the statement on the slave, a failure would occur,
because the Query_log_event was not transporting the expected
error code, but 0 instead.

This was because when the master was getting the error code to
set it in the Query_log_event, the executing thread would be
assumed to have been killed:
THD::killed==THD::KILL_BAD_DATA. This would make the error code
fetch routine not to check thd->main_da.sql_errno(), but instead
the thd->killed value. What's more, is that the server would
thd->killed value if thd->killed == THD::KILL_BAD_DATA and return
0 instead. So this is a double inconsistency, as the we should
not even check thd->killed but rather thd->main_da.sql_errno().

We fix this by extending the condition used to choose whether to
check the thd->main_da.sql_errno() or thd->killed, so that it
takes into consideration the case when:
thd->killed==THD::KILL_BAD_DATA.