message: improve not-found error message in list_directory_v2
  Errors that say "something went wrong" should provide
  context about what went wrong
diff: @@ -646,7 +646,10 @@ def list_directory_v2(path):
  errors.NotFoundError if directory doesn't exist
  """
  if not is_directory(path):
  - raise errors.NotFoundError(None, None, "Could not find directory")
  + raise errors.NotFoundError(
  + node_def=None,
  + op=None,
  + message="Could not find directory {}".format(path))
  with errors.raise_exception_on_not_ok_status() as status:
  # Convert each element to string, since the return values of the
  # vector of string should be interpreted as strings, not bytes.
  