Use ClientError as default exception on Django MailBackend.

[gcavalcante8808]

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.