When converting a QB query to SQL, prepared statement params aren't substituted

We don't substitute prepared statements parameters when converting to SQL, e.x.

``` sql
SELECT count(*) AS "count"
FROM "PUBLIC"."PRODUCTS"
WHERE "PUBLIC"."PRODUCTS"."CATEGORY" = ?
```

This results in a somewhat confusing `Parameter "#1" is not set` error.

Ideally we should substitute the actually parameters, but if we can't do that we should provide a more helpful warning of some kind.
