[frontend][python] print friendly error on http when https is enforced
    
    See #502
    
    Allow to set boolean field `encrypted` in the copr-cli config.
    By default, it is set to `True` and it is not suggested to
    a user to explicitly set it in the config. Only on instances,
    that don't enforce https (e.g. local docker-compose instance),
    it is.
    
    When the connection should be `encrypted` and `copr_url` is
    accidentally set to http:// URL, error is raised.