message: Reduced exception on encountering an unknown element to a warning.
diff: @@ -210,7 +210,7 @@ void checkElements(int line, std::vector<tinyxml2::XMLElement const*> const& ele
  {
  std::string value = e->Value();
  encountered[value]++;
  - check((required.find(value) != required.end()) || (optional.find(value) != optional.end()), e->GetLineNum(), "unknown element <" + value + ">");
  + warn((required.find(value) != required.end()) || (optional.find(value) != optional.end()), e->GetLineNum(), "unknown element <" + value + ">");
  }
  for (auto const& r : required)
  {
  