Handle a 501 in cert-find from dogtag as a "not supported"
    
    Upgrading from d9 -> d10 does not set up the RESTful interface
    in dogtag, they just never coded it. Rather than trying to backport
    things they have decided to not support upgrades.
    
    We need to catch this and report a more reasonable error. They are
    returning a 501 (HTTP method unimplemented) in this case.
    
    https://fedorahosted.org/freeipa/ticket/3549