[ZabbixAPI] Fix error message.
[DBClientZabbix] Fix constant value: EVENT_ID_NOT_FOUND

-1 ->
 0

When Hatohol is initial state, the ArmZabbixAPI::updateEvents() function
gets event data with dividing. But, in current implemantation,
the DBClientZabbix::getLastEventId() function doesn't return
EVENT_ID_NOT_FOUND. And, when Hatohol is initial state, the
DBClientZabbix::getLastEventId() function return "0".

So, I change the EVENT_ID_NOT_FOUND value.