Ensure drivers loaded before failing

so that deprecated are known
PLAT-132

@e0d plz test: this should fix your problem
any(@doctoryes @mulby @cpennington) plz review

Did red, green TDD
Since you've memoized `_drivers`, you can just move lines 225 - 227 out to before the `try` on 184 to pre-load the drivers, and then remove these lines.