When grader error occurs, stderr should be logged

[carsongee]

at https://github.com/edx/xqueue-watcher/blob/fd79253eb524c9e8d8d4d769107ae9c9dd85e113/xqueue_watcher/jailedgrader.py#L146 only stdout gets logged, but it would be very helpful to also see stderr since it often includes output like the exception trace or that the sudo user for the codejail isn't configured correctly.

I plan on writing the code to add it, I just wanted to note it here while I remember.