[FIX] mail: do not shadow SMTP errors with QUIT

When an exception is raised during the course of a normal SMTP session,
smtplib's `sendmail()` method will close the channel automatically,
so any further attempt at sending SMTP commands will fail with an
SMTPServerDisconnected error, even just calling `smtp.quit()`.
This will shadow the original error, and make debugging harder.

Fixes #1493