cert-request: raise error when request fails
    
    Fix a regression in recent change to request cert via Dogtag REST
    API.  'ra.request_certificate' was no longer raising
    CertificateOperationError when the cert request failed.  Inspect the
    request result to determine if the request completed, and raise if
    it did not.
    
    Fixes: https://fedorahosted.org/freeipa/ticket/6309
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>