SMTP::getLastReply() should be populated with last server reply…

…no matter what it was…

The SMTP::getLastReply() result is not populated in case of server error.
Truth be said, it should, for consistency's sake at the very least.
But real reason is that writing something like 

      if(!$inst->data($_message))
        throw new Exception('DATA command unsuccessful. Last server reply: ' . $inst->getLastReply());

is an order of magnitude cleaner, than a tantric dance with array values (you have to turn SMTP::getError() method call into variable somewhere first, too!)…

One day, someone have to turn all this into proper exceptions…