This change adds the error handling for Retry logic
in pgbk to also handle `pgerrcode.DuplicateTable`.
When two (or more) auth instances both try to create/setup
the same schema at nearly the same time, even with `IF NOT EXISTS`,
Postgres can still return `duplicate_table` (`42P07`) in a concurrent
create race. Retrying lets the losing instance re-check after the
winning instance finishes creating the table.