Degrade gracefully in case of initialization errors

[andreacampi]

If booting Treesaver fails for some reason (e.g. no grids for the current size etc), the current unload leaves the page empty and completely unusable.

It would be nice to handle this in a better way; i.e., unload should set a 'no-treesave' class on the html tag. If we got far enough to set OS and capabilities, it should leave them there.
This would allow us to provide CSS to handle this case in a similar way to what we do with no-js.