Change internal cpp11::stop() to C++ exceptions

[sbearrows]

Fixes #203

First attempt so let me know if this looks correct.

[jimhester]

These should throw some sort of classed exception object, e.g. one of https://en.cppreference.com/w/cpp/error/exception or a custom cpp11 exception class that derives from std::exception.

Possibly https://en.cppreference.com/w/cpp/error/invalid_argument would work for most of them, but you should use your judgement as to what is most appropriate for each case.

[sbearrows]

Okay thank you!

[jimhester]

LGTM now, you can merge this whenever you are done and the tests pass.