message: [c++] Catch exception by ref in attribute example
  Resolves a warning emitted by G++ 9.3.0
diff: @@ -92,7 +92,7 @@ int main()
  {
  bond::Apply(Validator(), obj);
  }
  - catch(bond::Exception e)
  + catch(const bond::Exception&)
  {
  // values in range
  obj.s.n = 13;
  