Propagate pending exception from cert_client_cb

There can be a pending exception in the cert_client_cb if the server
certificate failed verification and the server requested a client
certificate. Since the handshake is going to be terminated, just
return from client_cert_cb immediately indicating no client cert will be
provided, allowing the existing exception to propagate.

Bug: 3149826
git cherry-pick 30a77f316c03906ca59d6ebe5b6c7f0ff734aadb

Bug: 3184701
Change-Id: I58b038267f66d6b5f80e9f3d81ff1c0f8052ef27