Ensure 'no sender' exception is passed through event dispatcher

When using FailoverTransport, all Swift_TransportException are caught and their messages are never passed through the event dispatcher (https://github.com/swiftmailer/swiftmailer/blob/master/lib/classes/Swift/Transport/FailoverTransport.php#L57).
This PR addresses this issue by passing "Cannot send message without a recipient" exceptions within MailTransport and SmtpTransport through the event dispatcher, allowing them to be collected by plugins such as logging.

This will provide 'missing sender' information within plugins when using FailoverTransport.