Raise error on unsuccesful automation

[fgregg]

When doing a browser automation that is not successful, it would be really helpful to either raise an error, change the status code from 200 to something more indicative of an error, or both.

[Gallaecio]

You can do it through a relatively simple downloader middleware, and I don’t think it makes sense as a default, so I am not sure if there is much to do here. Maybe document it along with #118? And it would also cover #120.