Fix SSLv3 alert if no Client Ceritifcate sent after a request for one

In TLS if the server sends a CertificateRequest and the client does not
provide one, if the server cannot continue it should send a
HandshakeFailure alert. In SSLv3 the same should happen, but instead we
were sending an UnexpectedMessage alert. This is incorrect - the message
isn't unexpected - it is valid for the client not to send one - its just
that we cannot continue without one.

Reviewed-by: Emilia Käsper <emilia@openssl.org>