A bottoming function should have infinite arity

A bottoming function should have infinite arity

I can't think how this one escaped for so long, but
	(error "foo")
should have arityType ABot, just as 'error' itself does.

This improves eta expansion.  I spotted it when looking at the function

  Data.Array.Parallel.Arr.BBArr.writeMBB

in the ndp package.