Test {{foo bar}} throws for scalar foo

We can't test the case where `foo` is undefined, unfortunately, without changing `self.lookup` (which will change as part of "new this" anyway).

Also kill try/catches in UI.render and materialize.  They were a nice thought, but it's weird to not get an exception if you call these functions directly and they encounter an error.  Error-catching wrappers should either be right adjacent to app code (helpers, callbacks, etc.) or part of reactive re-rendering, not synchronous materialization.