Calls cls.transfer on all records in sources.

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