CustomStructure.update: raise KeyError for bad param names

This minor change makes it so that in the python bindings, `FLANNParameters.update` (and hence also `FLANNParameters.__init__`) raises a `KeyError` rather than silently ignoring if you give it a bad key name, just like `FLANNParameters.__setitem__`.

(I just noticed that this was masking a bug in my own code, which was mildly annoying.)