register_[pre_]shutdown_function never throw

[K-ballo]

The documentation for those functions state:

If this function is called before the runtime system is 
initialized, or while the shutdown functions are 
being executed, or after that point, it will raise a `invalid_status` 
exception.

Their implementations actually never throw. Ideally, these functions would correctly install the callbacks if called before the runtime system is initialized, and only throw an exception if called after the shutdown functions began executing.