message: Implement better to_string for expected<T>
diff: @@ -432,9 +432,9 @@ public:
  };
  template <class T>
  -auto to_string(const expected<T>& x) -> decltype(to_string(*x)) {
  +std::string to_string(const expected<T>& x) {
  if (x)
  - return to_string(*x);
  + return deep_to_string(*x);
  return "!" + to_string(x.error());
  }
  