Adds another key/value pair to log events. The new field
"subsystem" will contain one of the values "InnoDB" (for
errors thrown in the InnoDB storage engine proper), "Repl"
(for those thrown by the replication subsystem), or "Server"
(otherwise).

This value is also added to the traditional log. This field
will appear  between '[' <error_number> ']' and
'[' <error_text> ']'.