This event can be fired if remote server provided Banner text
before (successful) authentication was performed. When provided, it
usually contains information about the server, possible warning, or
any kind of data server administrator wants you to see. It can also
provide specific "challenge" information you should use when
providing the password.
If you ignore this event, same banner text will be provided as
first data received in
Received event. However, if you use this event and show received
Text, you should set it to empty string before event
completes, so data isn't duplicated again in Received event.