docs: def transfer_all(cls, field_mappings, sources, destination_factory=None):
      """Calls cls.transfer on all records in sources."""
      for index, source in enumerate(sources):
        try:
          yield cls.transfer(field_mappings, source, destination_factory or (lambda x: x))
        except Exception as ex:
          raise Exception("Error with source #{0}: {1}".format(index, ex)) from ex