[IMP] mail: store the reason when failing to send an email; usability improvements in configuration, now showing the number of failed emails + links for easier configuration.

Task: https://www.odoo.com/web?debug=#id=5051&view_type=form&model=project.task&action=333&active_id=131
Pad: https://pad.odoo.com/p/r.sPEEpRfkDZ32bf49
help="Failure reason. This is usually the exception thrown by the email server, stored to ease the debugging of mailing issues."