Problem:
On debug build, a failing FLUSH LOGS attempts to send OK to connection after
having sent error reply before. This is caused by missing error propagation
between where the error happened (ha_commit_trans inside close_table()) and
where the OK is sent (at the top level switch statement in
mysql_execute_command).

Fix:
Make previously void methods return bool for success/failure, and check for
errors along the way as necessary.