ensure 'no sender' exception is passed through event dispatcher (MailTransport and SmtpTransport)
feature #548 Ensure 'no sender' exception is passed through event dispatcher (TomK)

This PR was merged into the 5.3-dev branch.

Discussion
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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.

Commits
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0475194 ensure 'no sender' exception is passed through event dispatcher (MailTransport and SmtpTransport)