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
Depending on this test order, couldn't this actually not be giving us the coverage we feel like it should?

I'm far from a python unit testing expert, so some details are not totally clear to me (and critical in this case)...

When I was going to write a test for this... it seemed like another test could have loaded the module that sets the deprecated fallback, making this test pass even though production code would fail. This would happen because the call to `cls._separate_namespace()` would raise an InvalidKeyError which, in turn, would instantiate the deprecated fallback. In production code the module would never have been loaded, so the fallback would not be available.

I realize that a more recent change broke it more significantly... so I probably should have written this test anyway, but I am a bit concerned that it may give us a false sense of security.

Perhaps we should include a comment to that effect?