GREG-5: Switching exception type to avoid problem with mypy

greg/api/views/person.py

-from django.core.exceptions import BadRequest
+from django.core.exceptions import ValidationError
 from django_filters import rest_framework as filters
 
 from rest_framework import viewsets
@@ -45,6 +45,6 @@ class PersonRoleViewSet(viewsets.ModelViewSet):
 
         if person_id is None:
             # Should not happen, the person ID is part of the API path
-            raise BadRequest("No person id")
+            raise ValidationError("No person id")
 
         serializer.save(person_id=person_id)