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

kijiproject/kiji-express,kijiproject/kiji-express,kijiproject/kiji-express