Add better error handling throughout

[wibblymat]

There are a lot of TODOs for error handling, and a lot of places where a Promise could reject that I don't catch the errors.

In #30, Francois identified await record.save(); as something I do more than once without handling any errors.

After #31 lands, use the new pubsub system to surface some of these errors to the user. For others, try to have retries or something.