message: Improve the error messages in custom_export_strategy.
diff: @@ -51,6 +51,7 @@ def make_custom_export_strategy(name,
  feature_columns: A list of feature columns.
  export_input_fn: A function that takes no arguments and returns an
  `InputFnOps`.
  + use_core_columns: A boolean, whether core feature columns were used.
  Returns:
  An `ExportStrategy`.
  @@ -196,7 +197,7 @@ def convert_to_universal_format(dtec, sorted_feature_names,
  matching_id.int64_value = split.feature_id
  node.custom_left_child_test.Pack(categorical_test)
  else:
  - raise ValueError("Unexpected node type %s", node_type)
  + raise ValueError("Unexpected node type %s" % node_type)
  node.left_child_id.value = split.left_id
  node.right_child_id.value = split.right_id
  return model_and_features
  @@ -236,7 +237,7 @@ def _get_feature_importances(dtec, feature_names, num_dense_floats,
  assert tree_node.node_metadata.gain == 0
  continue
  else:
  - raise ValueError("Unexpected split type %s", node_type)
  + raise ValueError("Unexpected split type %s" % node_type)
  # Apply shrinkage factor. It is important since it is not always uniform
  # across different trees.
  sums[split_column] += (
  