Update the postgres-to-sqlite-export script to be a lot more cautious

[stevejalim]

We have a proof-of-concept script for generating a fresh sqlite DB from postgres, but we need to refactor it ahead of production use.

We need to ensure that:

1. it's purely additive on the sqlite side, rather than having to delete certain things after loading
2. it's atomic, so if any step fails, the whole thing is abandoned
3. if failure occurs, no DB is generated (not even an empty one)
4. if failure occurs, Sentry is notified, loudly