Delete Discord users if they've left the server.

Closes #968

allianceauth/services/modules/discord/tasks.py

@@ -5,7 +5,7 @@ from django.contrib.auth.models import User
 from django.core.exceptions import ObjectDoesNotExist
 from allianceauth.notifications import notify
 from celery import shared_task
-
+from requests.exceptions import HTTPError
 from allianceauth.services.hooks import NameFormatter
 from .manager import DiscordOAuthManager, DiscordApiBackoff
 from .models import DiscordUser
@@ -72,6 +72,15 @@ class DiscordTasks:
                 logger.info("Discord group sync API back off for %s, "
                             "retrying in %s seconds" % (user, bo.retry_after_seconds))
                 raise task_self.retry(countdown=bo.retry_after_seconds)
+            except HTTPError as e:
+                if e.response.status_code == 404:
+                    try:
+                        if e.response.json()['code'] == 10007:
+                            # user has left the server
+                            DiscordTasks.delete_user(user)
+                            return
+                    finally:
+                        raise e
             except Exception as e:
                 if task_self:
                     logger.exception("Discord group sync failed for %s, retrying in 10 mins" % user)