message: Raise error when opening user-defined config fails
diff: @@ -342,9 +342,14 @@ void print(const config_value::dictionary& xs, indentation indent) {
  error actor_system_config::parse(string_list args, std::istream& ini) {
  // Content of the INI file overrides hard-coded defaults.
  - if (ini.good())
  + if (ini.good()) {
  if (auto err = parse_config(ini, custom_options_, content))
  return err;
  + } else {
  + // Not finding an explicitly defined config file is an error.
  + if (auto fname = get_if<std::string>(&content, "config-file"))
  + return make_error(sec::cannot_open_file, *fname);
  + }
  // CLI options override the content of the INI file.
  using std::make_move_iterator;
  auto res = custom_options_.parse(content, args);
  @@ -516,6 +521,7 @@ error actor_system_config::extract_config_file_path(string_list& args) {
  auto evalue = ptr->parse(path);
  if (!evalue)
  return std::move(evalue.error());
  + put(content, "config-file", *evalue);
  ptr->store(*evalue);
  return none;
  }
  