message: Revise re-used slot variable dependency error message to be more informative as to the offending variable.
diff: @@ -119,8 +119,9 @@ def _serialize_slot_variables(trackable_objects, node_ids, object_names):
  if slot_variable in node_ids:
  raise NotImplementedError(
  "A slot variable was re-used as a dependency of a "
  - "Trackable object. This is not currently allowed. File a "
  - "feature request if this limitation bothers you.")
  + f"Trackable object: {slot_variable}. This is not currently "
  + "allowed. File a feature request if this limitation bothers "
  + "you.")
  checkpoint_name = naming_scheme(
  variable_path=object_names[original_variable],
  slot_name=slot_name)
  