fix(coreos-install): Don't abort after filtering partprobe errors.

Filtering out pointless errors from partprobe in efe90ac7 broke this
script, causing it to abort when there weren't any errors other than
what it filtered. Oops.

Reported in https://github.com/coreos/init/commit/efe90ac77f970e257e2313a05f1dd0e8ba9cd84e#commitcomment-6202472