message: Add unsupported_feature to exceptions.hpp
diff: @@ -14,4 +14,9 @@ struct instantiate_error : public std::runtime_error
  using runtime_error::runtime_error;
  };
  +struct unsupported_feature : public std::runtime_error
  +{
  + using runtime_error::runtime_error;
  +};
  +
  } // namespace fizzy
  