Viewpoint.go() does nothing, silently, when the viewpoint has no world #842

Viewpoint.go() returns without doing anything when the viewpoint has no world: if (!this.world) return; in packages/core/src/core/Viewpoints/src/viewpoint.ts. A few lines below, a wrong camera type throws a clear error, so the two cases behave differently.

The easy way to hit it: viewpoints created by BCFTopics.load take Viewpoints.world, which is null unless the app sets it. go() then resolves and the camera does not move, with no message. Reported by @cmartif in #826 (comment).

Steps (components on main):

const components = new OBC.Components();
const viewpoint = components.get(OBC.Viewpoints).create(); // Viewpoints.world is null
await viewpoint.go(); // resolves, nothing happens

Expected: an error that says the viewpoint has no world and how to give it one. updateCamera() and takeSnapshot() already report it by resolving false; go() returns Promise<void>, so the caller has no way to tell.