Support a not-ready state for zoomhub embeds.

- Fix bugs in the logic that detects whether a dzi has been written yet.
- Add the ability for the embed code to gracefully fail over to a 'queued' message when the dzi has not been written yet. Note that this 'queued' messsage is just a DZI that I made from a screencap of zoom.it in that state. Hokey, but it'll work for the time being, and I had only a couple of hours today. Maybe we can replace it with an animated gif of a stick figure with a shovel.
To be consistent with the rest of the codebase, e.g. `tileFormat`, `createTileSourceBlock`, etc., could we use `camelCase` here?