Bug 1639631 - Fix calls to subprocess.check_output.
Bug 1639631 - Fix calls to subprocess.check_output.

text=True can only be used in python3.7 and up, but we're on 3.6. So we should
use the equivalent universal_newlines=True argument instead. Also, when catching
errors thrown from subprocess.check_output, let unexpected errors propagate
so we get a stack in the log and generally more obvious failure. Keep catching
the "expected" failure (non-zero return code from process).

bill-mccloskey/searchfox,bill-mccloskey/searchfox,bill-mccloskey/searchfox,bill-mccloskey/searchfox,bill-mccloskey/searchfox,bill-mccloskey/searchfox