message: [contour] Report "Unknown action" through errorlog() rather than cerr.
diff: @@ -827,7 +827,7 @@ namespace
  auto actionOpt = actions::fromString(actionName);
  if (!actionOpt)
  {
  - cerr << "Unknown action: '" << _parent["action"].as<string>() << '\'' << endl;
  + errorlog()("Unknown action '{}'.", _parent["action"].as<string>());
  return nullopt;
  }
  