Add "error" event(s)?

@benstoltz I _think_ the only reason we added #197 was b/c we needed an error handler in Open Data, right? I mean, we're only using it to handle errors, not doing anything with the `spec` that's returned that's returned, right?

Another way (that I believe you suggested) to handle this would have been to add an "error" event, or "update-error". Similarly we could add "render-error" event to deal w/ any error returned to the callback sent to vega's `parse`. I also believe you said in vega 4 that parse no longer takes a callback.

So:

1. should we add these error events (if so I think we should deal w/ #239 first, b/c the error events would have the same issue)?
1. should we deprecate and eventually remove passing the callback to `show`/`update`/`_renderSpec`?