message: Print an error if actor_system_config::set fails
diff: @@ -310,7 +310,13 @@ actor_system_config& actor_system_config::set_impl(string_view name,
  return set_impl("middleman.app-identifiers", std::move(value));
  }
  auto opt = custom_options_.qualified_name_lookup(name);
  - if (opt != nullptr && opt->check(value) == none) {
  + if (opt == nullptr) {
  + std::cerr << "*** failed to set config parameter " << name
  + << ": invalid name" << std::endl;
  + } else if (auto err = opt->check(value)) {
  + std::cerr << "*** failed to set config parameter " << name << ": "
  + << to_string(err) << std::endl;
  + } else {
  opt->store(value);
  auto category = opt->category();
  auto& dict = category == "global" ? content
  