[helenkwok]

Fixes #842.

Viewpoint.go() now throws when the viewpoint has no world, with a message that says to set world on the viewpoint or Viewpoints.world before creating or loading viewpoints (for example with BCFTopics.load). This matches the existing error a few lines below for a camera that is not an OrthoPerspectiveCamera, and is added to the method's @throws docs.

updateCamera() and takeSnapshot() are unchanged: they already tell the caller by resolving false.

Test: src/core/Viewpoints/viewpoint-go.test.ts creates a viewpoint while Viewpoints.world is null and expects go() to reject. It fails on main ("promise resolved undefined instead of rejecting") and passes with this change. eslint is clean on both files.

Behaviour change to note: code that called go() on a viewpoint without a world and relied on it doing nothing will now get a rejected promise.