This version does not use transactions, but individual statements. As we have
unique constraints on the target tables for the inserts, we can just ignore
uniqueness violations there (as long as we always insert the same batch size, in
the same order).

This means the spec now must use truncation, not a transaction, as the
uniqueness violation means that the whole transaction for that spec would be
invalid, which isn't what we'd want. In real-world use, this isn't run in a
transaction anyway.

This commit also wraps unhandled exceptions, for easier finding in Sentry, and
logs with a consistent format, for easier searching.