Authentication exceptions during consumer metadata updates may not get propagated

The consumer should propagate authentication errors to the user. We handle the common case in ConsumerNetworkClient when the exception occurs in response to an explicitly provided request. However, we are missing the logic to propagate exceptions during metadata updates, which are handled internally by NetworkClient. This logic exists in ConsumerNetworkClient.awaitMetadataUpdate, but metadata updates can occur outside of this path. Probably we just need to move that logic into ConsumerNetworkClient.poll() so that errors are always checked.