Fix error reporting

[raquelxmoss]

Expected behaviour: Removing DOM: Observable.just(div('.hello-world', 'Hello world!')) and returning an empty function renders a blank results container.

Actual behaviour: Removing DOM: Observable.just(div('.hello-world', 'Hello world!')) continues to display "Hello World" in the results container.

To replicate: Write an empty function in the editor.

[Widdershin]

Yeah, I think that's because we broke the error reporting last night, and if you don't return an object with a key of DOM and a DOM observable stream as the value, it doesn't work.

[Widdershin]

I think this is actually about errors that start up after the application is run but don't happen synchronously. We would need something like a .catch in every driver's stream to get all the errors. Something to talk to staltz about perhaps