Rollup merge of #159425 - Joel-Wwalker:157049-nul-error-context, r=Darksonn

Windows: add context when opening NUL for child stdio fails

Opening `\\.\NUL` for `Stdio::Null` can fail in some environments, and `Command::spawn`/`output()` passes the raw OS error through. In the reported case that was `NotFound`, which looks exactly like the program itself being missing. Add context saying what failed to open and for which stream:

```
failed to open NUL device for child stdin: The system cannot find the file specified. (os error 2)
```

The `ErrorKind` is kept. `raw_os_error()` on this path becomes `None` since the payload is now a message string. I don't see a way to make opening NUL fail in CI, so no test; verified by hand in a local build by pointing the open at a bogus device name and checking the error out of `Command::output()`.

Fixes rust-lang/rust#157049