Read messages from the alerts.log file. Returns the number of messages in 'Alerts'
and an array of those messages in 'Messages'.
'LastAlert' contains the last message added to the alerts.log file.
This may be called periodically to get only recently added alerts.
The first time GetAlerts is called, the caller should set LastAlert="".
The caller may then retain the returned value and pass that LastAlert back
in subsequent calls, to only retrieve messages added since then.