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
Right, I'm assuming `drivers[namespace]` against the new cache is as performant as the old cache `O(1)` and that handling `KeyError` v caching and raising the error is as performant.