Raise specific exceptions on HTTP errors instead of RuntimeError

[TomAugspurger]

Mentioned in #603 (comment), when we throw an exception while reading a remote file in C++, we currently get a generic RuntimeError in python.

We should be able to translate these to more specific errors (HTTPError, TimeoutError, etc.). Doing a full exception hierarchy like httpx may be a bit much, but at least differentiating between a completed HTTP request with a 500 status code and a timeout is probably worthwhile.