improve protocol change error message
improve protocol change error message

simon-weber/Unofficial-Google-Music-API,tanhaiwang/gmusicapi,simon-weber/gmusicapi,peetahzee/Unofficial-Google-Music-API,jimyx17/gmusic,TheOpenDevProject/gmusicapi,thebigmunch/gmusicapi,dvirtz/gmusicapi,nvbn/Unofficial-Google-Music-API,dvirtz/gmusicapi