Use ClientError as default exception on Django MailBackend.#177

Hi Folks, Is It possible to hold the error_code that comes from postmark int he `smtp_code` attr? In this way, the postmark exception behaves more like an smtp_exception. What do you think? Or if we don't want to change the purpose of smtp_code (after all its a non RFC/SMTP error at all, just some api error_codes) , maybe we can put an `error_code` attr instead. If you consider this a useful sugestion I can make a PR for that. Thanks.