docs: def _validate_storage(storage, service_name, add_error):
      """Lazily validate the storage constraints, ensuring that they are a dict.
  
      Use the given add_error callable to register validation error.
      """
      if storage is None:
          return
      if not isdict(storage):
          msg = 'service {} has invalid storage constraints {}'.format(
              service_name, storage)
          add_error(msg)