Differentiate between StaleRevision error to other manager errors

Change-Id: Ia176520504a1e2536fd839c9fe0cf5ac4d853a5d

vmware_nsx/nsxlib/v3/dfw_api.py

@@ -126,6 +126,8 @@ def add_nsgroup_member(nsgroup_id, target_type, target_id):
     members = {'members': [member_expr]}
     try:
         return _update_nsgroup_with_members(nsgroup_id, members, ADD_MEMBERS)
+    except nsx_exc.StaleRevision:
+        raise
     except nsx_exc.ManagerError:
         # REVISIT(roeyc): A ManagerError might have been raised for a
         # different reason, e.g - NSGroup does not exists.