[ADD] mail_tracking_mailgun: manage failed state from mailgun

This change let odoo process a state sent from mailgun legacy webhooks
that seems to apply when the message is not sent because the related
email has been mark us as spam or have bounced before. For solve this
add two new states to _mailgun_event_type_mapping_mailgun_event_type_mapping
method:

* failed: Mailgun could not deliver the email to the recipient email server
* rejected: Mailgun rejected the request to send/forward the email

source in https://documentation.mailgun.com/en/latest/quickstart-events.html#events.

mail_tracking_mailgun/models/mail_tracking_email.py

@@ -44,6 +44,8 @@ class MailTrackingEmail(models.Model):
             'bounced': 'hard_bounce',
             'dropped': 'reject',
             'accepted': 'sent',
+            'failed': 'error',
+            'rejected': 'error',
         }
 
     def _mailgun_event_type_verify(self, event):