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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 request_error().

In CLI mode, this is usually the process STDERR.