Clarify/improve exception handling with QFuture

`QFuture` seems to struggle a bit with exceptions that do not derive from `QException` (documented, is this still necessary, technically?) and especially nested exceptions.

I'm not aware how relevant the latter is, just at the moment `std::throw_with_nested` and `std::rethrow_if_nested` are not really usable due to the exception cloning I assume. Maybe this should be documented or fixed eventually, examples attached.