HTTPX hides missing CA certs

[nhoad]

Hi! I recently hit upon a situation where my code was failing to negotiate TLS after the process had been running for some time. Long story short it's because I deploy my code to versioned paths, and once the old CA cert path is cleaned up, there's no CA certs to verify against.

In digging into this I realised I would have expected a FileNotFoundError to bubble up through httpx, and I found a bug in the httpx code that sets up verification - https://github.com/encode/httpx/blob/master/httpx/_config.py#L140-L145. If this block had an else clause to raise an exception about not knowing how to load the ca_bundle_path, it would have been significantly easier to debug this.