Handling exceptions in callbacks

[szhorvat]

This is a reminder to consider a feature once this library gets mature enough.

std::current_exception() and std::rethrow_exception() are helpful in catching an exception thrown within a callback passed to a C igraph function, storing it, and rethrowing it once the C function has returned.

See: https://en.cppreference.com/w/cpp/error/current_exception

igraph-cpp should provide features to make this very easy.