Exit code 134 not propagated

I recently switched a project from using `npm-run-all` to using `&&` because we found that `npm-run-all` was returning exit code 0 when one of its several sub-tasks running webpack ran out of memory, causing it to return exit code 134. Since it returned exit code 0, it didn't fail the CI processes, so it took a few days for our QA team to discover the problem.
