fix avconv error handling flow
fix avconv error handling flow

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