Use the proper specific permission

aleksis/core/schema/base.py

 from django.db.models import Model
+from django.core.exceptions import PermissionDenied
 
 import graphene
 from graphene_django import DjangoObjectType
@@ -45,4 +46,4 @@ class DeleteMutation(graphene.Mutation):
             obj.delete()
             return cls(ok=True)
         else:
-            raise Exception("Permission denied")
+            raise PermissionDenied()