As can be seen here:
Created attachment 88950 [details]
See dependent bug for the follow up to remove the fixme (and do a better fix).
Comment on attachment 88950 [details]
Committed as http://trac.webkit.org/changeset/83398
From looking at the python code implementing subprocess, it looks like either os.waitpid() could be returning something weird, or it could be raising os.error.
So, something's definitely wrong, but having NRWT fall over at that point isn't all that useful, so your change is good. I will see if I can follow-up and reproduce it in bug 58202.