Łukasz, Martin - I'm not sure how to proceed here, of course both ways out are reasonable. I'd be happy to provide (however small) patch for either one (adding Error to __all__ or just adding test for __all__). :)

My inner librarian would of course either push the Error to fully become part of public API or change its name to non-public _Error or something, but I'd much rather rely on you in this regard.