Improve SMTP error handling
Currently, Geary will display a problem report when sending an email fails, but without specifically saying what was wrong. In the case of a specific problem with the outgoing message (invalid sender, recipients, message is too big) Geary should be specific about that. Further, if sending an email fails for one of these reasons, it shouldn't block sending others in the queue.
Some research and design work is needed here about what classes of errors to report and how to prompt the user about it.
See also #352