Problem:
========
my_message function called from within a plugin was not capable of aborting
MYSQL_AUDIT_SERVER_STARTUP_STARTUP event. my_message function sets error state of the THD
associated with the generated event, but THD was not available (NULL) during
MYSQL_AUDIT_SERVER_STARTUP_STARTUP event generation.

Fix:
====
THD is created during MYSQL_AUDIT_SERVER_STARTUP_STARTUP event generation. It holds
information whether the event generation ends with the error state as a result of calling
my_message function.

Additionally the message is added to the error log, which covers this bug:
Bug #22142166 MYSQL_AUDIT_SERVER_STARTUP_CLASS EVENT TERMINATION SHOULD IMPROVE DIAGNOSTICS

Reviewed-by:
============
Georgi Kodinov <georgi.kodinov@oracle.com>
Ramil Kalimullin <ramil.kalimullin@oracle.com>