If the cafile is not present or readable then raise an exception
    
    This can happen on the API level if a user passes in None as
    cafile or if the value passed in does not exist or is not
    readable by the IPA framework user.
    
    This will also catch situations where /etc/ipa/ca.crt has
    incorrect permissions and will provide more useful information
    than just [Errno 13] Permission denied.
    
    https://pagure.io/freeipa/issue/7145
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>