Abort individual scripts when steps fail

[evanw555]

Similar to #13, except for the individual build and deploy scripts. That linked PR adds set -e to the beginning of the root entrypoint.sh script, but we should also do the same for the individual tasks.

My use case uses the root one for building and deploying on push, but uses the build-only script for testing PRs (thus, having this abort setting is crucial to testing PR builds).

It's a simple change, I'll likely just end up opening a PR myself.