Bounce messages from Exchange 2003 not parsed

We had a problem with bounce messages from Exchange (2003) not being processed by Civi and found that the civicrm_mailer_event_bounce() function wasn't getting the message body. The parsed out message text was null.

The attached patch checks if the parsed text is null and if so relies on the email subject and status (error code) to detect a hard bounce (error 5.1.1) from Exchange.