Document what responses result in exceptions

Suggest that the documentation be more explicit on what responses result in exception.

In particular, what status codes result in an exception? What statuses are the responsibility of client code?

I see a lens for `checkResponse` that I suspect gives some control. But in `defaults` the value for it is `Nothing` and yet `get` throws exceptions on non-200 statuses. I can't find much documentation of that option, either.