[IMP ]add failure_reason field and counting number of fail mail

[IMP] improve help and change field name fail_mail to fail_counter and change method name of counting fail mail
Merge pull request #936 from odoo-dev/master-send-failmail-warning-avo

[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.