Returns path to where checksums are stored for a given dataset.

def _get_path(dataset_name):
  """Returns path to where checksums are stored for a given dataset."""
  path = _checksum_paths().get(dataset_name, None)
  if path:
    return path
  msg = ('No checksums file could be find for dataset %s. Please create one in '
         'one of: %s') % (dataset_name, ', '.join(_CHECKSUM_DIRS))
  raise AssertionError(msg)