When "ArcanistRuntime" exits with a nonzero exit code, emit that exit code

Summary:
See https://discourse.phabricator-community.org/t/failed-arc-patch-does-not-return-non-zero-exit-code/3584.

Returning an integer from a top-level PHP file doesn't actually affect the process exit code, as much as I might wish it does.

Test Plan: Ran arc patch adflsnadfsln; echo $?, saw a nonzero exit code after this fix.

Differential Revision: https://secure.phabricator.com/D21037