Include stderr in error reports for easier debugging

[JasonGross]

On top of #317
Fixes #318

Except I think it doesn't quite work in the current form

[JasonGross]

@Zimmi48 Do you have any opinion on the implementation here? I've checked that it still emits stderr and stdout in the log, doesn't crash, etc.

[Zimmi48]

I can't say that I understand precisely all the code (in particular the copy_stream function), but this looks good to me.

[JasonGross]

I can't say that I understand precisely all the code (in particular the copy_stream function), but this looks good to me.

copy_stream reads from a steam character by character, simultaneously saving it to a buffer and writing it to another stream. It was written mostly by claude.ai. It's possibly a bit overkill (we could just read from stdout and stderr all at once), but this version keeps them interleaved in the logs.