message: remove extra const
diff: @@ -771,7 +771,7 @@ try
  {
  return this->options.at(name);
  }
  -catch (const std::out_of_range const& e)
  +catch (std::out_of_range const& e)
  {
  std::ostringstream msg;
  msg << "no option named \"" << name << "\" in parser_results." << e.what();
  