world: Raise a different exception for impossible chunk lookups.

Some tests fail, but it's hard to see which ones. Y'know why? Because some of
the tests are putting random crap into the reactor and causing other tests to
fail later! Whoo.

Refs #423.
utilities/coords: Isolate iterneighbors(), and test.

Refs #423.
plugins/physics: Use iterneighbor().

Refs #423.
utilities/coords: Isolate itercube() and test.

Refs #423.
plugins/physics: Refactor to use itercube().

Refs #423.