message: throws if can't generate matrix data
diff: @@ -57,9 +57,12 @@ struct DefaultSystemGenerator {
  if (config.HasMember("filename")) {
  std::ifstream in(config["filename"].GetString());
  return gko::read_generic_raw<ValueType, IndexType>(in);
  - } else {
  + } else if (config.HasMember("stencil")) {
  return generate_stencil<ValueType, IndexType>(
  config["stencil"].GetString(), config["size"].GetInt64());
  + } else {
  + throw std::runtime_error(
  + "No known way to generate matrix data found.");
  }
  }
  @@ -160,10 +163,13 @@ struct DistributedDefaultSystemGenerator {
  if (config.HasMember("filename")) {
  std::ifstream in(config["filename"].GetString());
  return gko::read_generic_raw<value_type, index_type>(in);
  - } else {
  + } else if (config.HasMember("stencil")) {
  return generate_stencil<value_type, index_type>(
  config["stencil"].GetString(), comm, config["size"].GetInt64(),
  config["comm_pattern"].GetString() == std::string("optimal"));
  + } else {
  + throw std::runtime_error(
  + "No known way to generate matrix data found.");
  }
  }
  