kiji-express: EXP-463. express.py script should exit with non-zero exit code if the command fails

Make sure subprocess exit codes are correctly propagated.

issue: https://jira.kiji.org/browse/EXP-463
review: https://review.kiji.org/r/2049
commit: https://github.com/kijiproject/kiji-express/commit/b2a9f852c256f1f8ee9a4643da7d1c7d2ae49188