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.
When this method is called without an event (a.k.a. `event=None`), this method produces the following error:
```
Traceback (most recent call last):
File "/opt/odoo/auto/addons/mail/models/mail_mail.py", line 278, in send
res = IrMailServer.send_email(msg, mail_server_id=mail.mail_server_id.id)
File "/opt/odoo/auto/addons/mail_tracking/models/ir_mail_server.py", line 88, in send_email
tracking_email.smtp_error(self, smtp_server_used, e)
File "/opt/odoo/auto/addons/mail_tracking/models/mail_tracking_email.py", line 213, in smtp_error
self.sudo()._partners_email_bounced_set('error')
File "/opt/odoo/auto/addons/mail_tracking/models/mail_tracking_email.py", line 203, in
_partners_email_bounced_set
]).email_bounced_set(self, reason, event=event)
File "/opt/odoo/auto/addons/mail_tracking_mailgun/models/res_partner.py", line 22, in
email_bounced_set
self._email_bounced_set(reason, event)
File "/opt/odoo/auto/addons/mail_tracking_mailgun/models/res_partner.py", line 33, in
_email_bounced_set
event['Message-Id'] or '') TypeError: 'NoneType' object has no attribute '__getitem__'
```
So, we now assume we do not always have an event.
