QDBusReply<T> essentially models std::expected<T, QDBusError>, so make sure it has the same interface as std::expected, so we can port to that come Qt 7 to gain std::expected's size improvements (union instead of struct), maybe even make QDBusReply an alias for std::expected.

The most obvious shortfall is that value() is not overloaded on const/non-const rvalue/value, lack of op* and operator bool()