_sent_failed cut the errback chain in MailSender

[aplanas]

MailSender._sent_failed return None, instead of failure. This cut the errback call chain, making impossible to detect in the code fail in the mails in client code.

[felipeboffnunes]

Well this is fair bug, and simple to fix (I guess). How should we go forward with it, though?
We return/raise the failure (not sure which, design-wise) but this is not backwards-compatible. What other things must we do around the fix to deal with the lack of compatibility between versions?
@Gallaecio @wRAR

[Prathm-s]

Hello @aplanas, I am interested in working on this issue, Could you please assign it to me?