chore(): test and validation improvements

src/backend/apps/reviews/services.py

@@ -1,5 +1,6 @@
 from typing import Any
 
+from config.errors import ConflictError
 from django.core.exceptions import ValidationError
 from django.db import transaction
 
@@ -91,13 +92,15 @@ def approver_group_create(
 ) -> ApproverGroup:
     _validate_experimenter(experimenter)
     if ApproverGroup.objects.filter(experimenter=experimenter).exists():
-        raise ValidationError(
-            {
-                "experimenter": (
-                    f"An approver group already exists for "
-                    f"experimenter '{experimenter.username}'."
-                )
-            }
+        raise ConflictError(
+            ValidationError(
+                {
+                    "experimenter": (
+                        f"An approver group already exists for "
+                        f"experimenter '{experimenter.username}'."
+                    )
+                }
+            )
         )
     approvers: list[User] = []
     if approver_ids: