Don't use the tempfile module in drm_ge

[mdpearson]

Turns out subprocess.check_output() is almost exactly what we need here, except that it drops stderr on the floor. I copied its implementation and added stderr support to CalledProcessError to make our use case run more smoothly. No more file i/o.

If this looks good I'll apply the same fixes to drm_slurm.

[egafni]

Looks great to me, and a nice general function to have to boot