messages: emit pubkey failures to callbacks

With this change, errors encountered while processing public keys
in 'ssh_packet_userauth_request' will be propagated back up to the
user-provided 'auth_pubkey_function' callback.

For example, if signature validation fails, the user callback will
be invoked with a signature state of SSH_PUBLICKEY_STATE_WRONG.

Before this change, a failure in signature validation would not be
made explicit via the 'ssh_server_callbacks'.