Use nested transaction to handle integrity error

The notes in the "avoid catching exceptions" callout here
(https://docs.djangoproject.com/en/5.0/topics/db/transactions/#controlling-transactions-explicitly)
say to do something like this to isolate error handling without stepping
on the operation of the transactions themselves.

dandiapi/zarr/views/__init__.py

@@ -139,13 +139,14 @@ def create(self, request):
         if dandiset.embargo_status != Dandiset.EmbargoStatus.OPEN:
             raise ValidationError('Cannot add zarr to embargoed dandiset')
         zarr_archive: ZarrArchive = ZarrArchive(name=name, dandiset=dandiset)
-        try:
-            with transaction.atomic():
-                zarr_archive.save()
+        with transaction.atomic():
+            try:
+                with transaction.atomic():
+                    zarr_archive.save()
+            except IntegrityError as e:
+                raise ValidationError('Zarr already exists') from e
 
-                audit.create_zarr(dandiset=dandiset, user=request.user, zarr_archive=zarr_archive)
-        except IntegrityError as e:
-            raise ValidationError('Zarr already exists') from e
+            audit.create_zarr(dandiset=dandiset, user=request.user, zarr_archive=zarr_archive)
 
         serializer = ZarrSerializer(instance=zarr_archive)
         return Response(serializer.data, status=status.HTTP_200_OK)