Figure out how to handle non-document navigations

[jgraham]

The last two cases in https://html.spec.whatwg.org/#process-a-navigate-response don't have any bidi integration points; we need to figure out what should happen from a bidi point of view when we end up there. The way things stand a navigate command will never return in those cases, but it's also not clear that an error happened. This is complicated by the fact that in the last case the browsing context didn't navigate at all; maybe we should have a download event with the same navigation id?