Freak out about Neighbourhood objects with id == 'neighbourhoods'

I'm not super happy with the way the exception is written, but it's
specifically exceptable, has a useful error message and is easily
tested, so from a user's perspective, it should be completely adequate.

cc. #1
Fix Python 3 ImportError.

cc. #1
Indent NeighbourhoodsNeighbourhoodException.__init__ less grossly.