Remove NSPRError exception from platform tasks
    
    ipalib.x509 no longer raises NSPRError. PyCA cryptography raises
    ValueError for invalid certs.
    
    https://fedorahosted.org/freeipa/ticket/5695
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>