Fix acceptor_rejects_untrusted_client_certificates following hyper upgrade

[jarhodes314]

In #3371, I ignored acceptor_rejects_untrusted_client_certificates since I wasn't getting the desired error message, but I believe the underlying functionality is still behaving as expected. The point of checking the error is correct is intended to ensure the process failed in the correct way. If we didn't do this, the client could, for example, fail to trust the server certificates and the connection would be immediately rejected rather than failing on a client certificate error. The type of error we get is also important as when working, it makes it easy for clients to diagnose the issue on their side rather than just getting some generic TLS failure which is near impossible to debug. The issue may well be caused by some refactoring on reqwest's side, as I had to change some error handling there.