This is available as
IO\request_errorx in the www repository.
Return the error output handle for the current request
namespace HH\Lib\IO; function request_errorx(): CloseableWriteFDHandle;
This is usually only available for CLI scripts; it will fail with `EBADF. in most other contexts, including HTTP requests.
For a non-throwing version, use
In CLI mode, this is usually the process STDERR.