Investigate possibility of error handling in QFuture without using exceptions

Using exceptions doesn't always fit everyone's programming style. Investigate what approaches can be used to provide an alternative solution. One possibility is using something similar to std::expected, which may be useful to have in general, not only for QFuture. There was a discussion on this topic here: https://lists.qt-project.org/pipermail/development/2020-March/039090.html