ipaclient: schema cache: Handle malformed server info data gracefully
    
    As a part of CLI schema cache some data about each previously contacted server
    are stored in simple JSON file. The file may get corrupted and became
    undecodable for various reasons (parallel access, file system error,
    tampering). Since the data are not necessary we should just warn an continue.
    
    https://fedorahosted.org/freeipa/ticket/6578
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>