message: Fix `to_string` for `expected<void>`
diff: @@ -438,6 +438,12 @@ std::string to_string(const expected<T>& x) {
  return "!" + to_string(x.error());
  }
  +inline std::string to_string(const expected<void>& x) {
  + if (x)
  + return "unit";
  + return "!" + to_string(x.error());
  +}
  +
  /// @cond PRIVATE
  /// Assigns the value of `expr` (which must return an `expected`)
  /// to a new variable named `var` or throws a `std::runtime_error` on error.
  