[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
failure_reason: _('Error without exception. Probably due do sending an email without computed recipients.')