`boost::process::child`: `std::error_code` versus `exit_code()`

Most of the `boost::process::child` API provides `std::error_code` values, but it's only possible to get the final process exit code as `int`. What is the reason for this dichotomy?

Is there any overlap, or is it that the `std::error_code` is only for reporting errors that occur during Boost/C++/OS processing, while `exit_code()` is only for reporting the exit code returned by a child process that exited of its own accord?