Add ServerError exception

[jacobtomlinson]

Closes #118.

When we get an httpx.HTTPStatusError that we can't recover from there is usually a lot of information about what went wrong in the body of the response. This PR reraises that exception as a kr8s.ServerError which exposes the response status directly.

>>> from kr8s.objects import Pod
>>> pod = Pod(invalid_pod_spec)
>>> pod.create()
ServerError: Pod "my-pod" is invalid: spec.containers[0].name: Required value

This is much more useful that the previous HTTPStatusError: Client error '422 Unprocessable Entity' for url 'https://127.0.0.1:37237/api/v1/namespaces/default/pods' exception.

The ServerError exception also has a .status attribute with the full server-side status object and a .response attribute with the httpx.Response object.