Root cause: in the current tree, mach commands in mozilla-central output a
warning to stderr. The substitute-local-geckoview script was calling a mach
command and combining the stdout and stderr streams of the process, parsing
it as JSON. This warning is not JSON so the script crashed.

The crashing code was copied from settings.gradle:
  https://searchfox.org/mozilla-central/rev/b70bc09685763c44a8e56e4e04cb741fa020701a/settings.gradle#26

The code in settings.gradle does an intuitive thing - capture stderr separately
but only print it on subprocess non-zero exit value - so we also copy that
solution. I'm not sure what an appropriate place to store code shared between
these two files would be so I didn't try to deduplicate it.

Differential Revision: https://phabricator.services.mozilla.com/D137591