client: raise KeyError from __getitem__ (BC) object.__getitem__ is supposed to raise either IndexError or KeyError (depending on whether the thing is a sequence or a mapping). Before, we would raise ValueError because that's what the context constructor raises. I choose to raise KeyError because IndexError felt a bit too limiting. This does sacrifice some magic with for loops handling IndexError. However, iteration of this object should be handled by a custom __iter__, so I don't think this is a problem.