crdb is a wrapper around the logic for issuing SQL transactions which performs
retries (as required by CockroachDB).
Note that unfortunately there is no generic way of extracting a pg error code;
the library has to recognize driver-dependent error types. We currently support
github.com/jackc/pgconn (which is used by
github.com/jackc/pgx/v4; previous pgx versions are not supported).
Note for developers: if you make any changes here (especially if they modify public APIs), please verify that the code in https://github.com/cockroachdb/examples-go still works and update as necessary.