Some fixes

This removes one crasher (if you didn't pass an error to `-rollbackToSavePointWithName:`) and a FIXME (how save point names are generated and passed to SQLite).

To be frank, I actually :

1) wrote my own wrapper around `-beginTransaction`/`-commit`/`-rollback`,
2) noticed that SQLite doesn't nest transactions,
3) learned of save points,
4) wrote another wrapper,
5) discovered `-inSavePoint:` existed.

My own save point version used a `CFUUIDRef`, and I can switch it here if you're interested.