Provide a better error message when deleting nonexistent attributes
    
    If --delattr is used on an attribute that's not present on an entry,
    and --{set,add}attr isn't being used on that same attribute,
    say that there's "no such attribute" instead of "<attribute> does
    not contain <value>".
    
    https://fedorahosted.org/freeipa/ticket/2699