[FIX] mail_tracking_mailgun: wrong failure event mappings

Before this patch, both `failed` and `rejected` mailgun events were mapped to `error` state.

However, that was wrong because [`mail_tracking` doesn't have an `error` state][1]. So, `rejected` is now translated into `reject`, and `failed` is translated into `hard_bounce` or `soft_bounce` depending on the severity, as specified by [mailgun docs][2].

Of course, to know the severity, now the method to obtain that info must change, it' can't be a simple dict anymore.

[1]: https://github.com/OCA/social/blob/f73de421e28a43d018176f61725a3a59665f715d/mail_tracking/models/mail_tracking_event.py#L31-L42
[2]: https://documentation.mailgun.com/en/latest/api-events.html#event-types

mail_tracking_mailgun/models/mail_tracking_email.py

@@ -32,10 +32,16 @@ def _mailgun_mandatory_fields(self):
         return ('event', 'timestamp', 'token', 'signature',
                 'tracking_email_id', 'odoo_db')
 
-    @property
-    def _mailgun_event_type_mapping(self):
-        return {
-            # Mailgun event type: tracking event type
+    @api.model
+    def _mailgun_event2state(self, event, default="UNKNOWN"):
+        """Return the ``mail_tracking`` equivalent event
+
+        Args:
+            event: Mailgun event response from API.
+            default: Value to return when not found.
+        """
+        # Mailgun event type: tracking event type
+        equivalents = {
             'delivered': 'delivered',
             'opened': 'open',
             'clicked': 'click',
@@ -44,14 +50,17 @@ def _mailgun_event_type_mapping(self):
             'bounced': 'hard_bounce',
             'dropped': 'reject',
             'accepted': 'sent',
-            'failed': 'error',
-            'rejected': 'error',
+            "failed": (
+                "hard_bounce" if event.get("severity") == "permanent" else "soft_bounce"
+            ),
+            "rejected": "reject",
         }
+        return equivalents.get(event.get("event"), default)
 
     def _mailgun_event_type_verify(self, event):
         event = event or {}
         mailgun_event_type = event.get('event')
-        if mailgun_event_type not in self._mailgun_event_type_mapping:
+        if self._mailgun_event2state(event) == "UNKNOWN":
             _logger.error("Mailgun: event type '%s' not supported",
                           mailgun_event_type)
             return False
@@ -194,8 +203,7 @@ def event_process(self, request, post, metadata, event_type=None):
                 res = 'OK'
         if res == 'OK':
             mailgun_event_type = post.get('event')
-            mapped_event_type = self._mailgun_event_type_mapping.get(
-                mailgun_event_type) or event_type
+            mapped_event_type = self._mailgun_event2state(post, event_type)
             if not mapped_event_type:  # pragma: no cover
                 res = 'ERROR: Bad event'
             tracking = self._mailgun_tracking_get(post)
@@ -251,8 +259,9 @@ def action_manual_check_mailgun(self):
                         [('mailgun_id', '=', item["id"])]) and (
                             item.get("recipient", "") ==
                             email_split(tracking.recipient)[0]):
-                    mapped_event_type = self._mailgun_event_type_mapping.get(
-                        item["event"], item["event"])
+                    mapped_event_type = self._mailgun_event2state(
+                        item, item["event"]
+                    )
                     metadata = self._mailgun_metadata(
                         mapped_event_type, item, {})
                     tracking.event_create(mapped_event_type, metadata)