In case of errors - write log & throw an exception

[ihor-sviziev]

Right now if I put incorrect credentials - nobody will know that emails aren't sent. In case of errors need throw an exception like this: https://github.com/magepal/magento2-gmailsmtpapp/blob/master/Model/Transport.php#L43

Note: Right now we don't have any exception if there returned incorrect status.