Document function that throws errors

[lrobidou]

Hi!
I've read the README of this project to document the rust parser.
I also read a bit of the C++ code and it appears some functions can throw an error but this is not documented.
For instance: Kff_reader::Kff_reader calls Kff_file::Kff_file which calls Kff_file::open. This can throw std::runtime_error, but can also crash (exit(1)). This makes it difficult/impossible to exit gracefully on the client side.
I have no idea how to easily document that in C++ :(