Yeah, I'm surprised they mentioned using longjmp for "exceptions". I think that just shows the age of the nethack codebase - maybe when it was being written people were trying to find ways of incorporating those error handling ideas into C. I doubt any modern C programmers would seriously consider using it over just returning an error code.
It's a common idiom, specifically in modern C code. See "C Interfaces and Implementations", one of the best books on the subject of writing reusable C. The author dedicates an entire chapter to exception handling, with longjmp as the primarily facility for it.
Error codes and longjmp are complementary methods of exception handling. Error codes can't handle every circumstance (well, at least not without undue pain). For example, error codes don't deal with nonblocking i/o or coroutines very well.
