cli: Don't crash when failing to list listening ports.
cli: Don't crash when failing to list listening ports.

Resolves #790.

Klaudit/livestreamer,intact/livestreamer,ethanhlc/streamlink,flijloku/livestreamer,okaywit/livestreamer,caorong/livestreamer,charmander/livestreamer,derrod/livestreamer,streamlink/streamlink,Feverqwe/livestreamer,wlerin/streamlink,jtsymon/livestreamer,programming086/livestreamer,wlerin/streamlink,lyhiving/livestreamer,bastimeyer/streamlink,melmorabity/streamlink,lyhiving/livestreamer,derrod/livestreamer,ethanhlc/streamlink,javiercantero/streamlink,chhe/livestreamer,programming086/livestreamer,Feverqwe/livestreamer,Dobatymo/livestreamer,mmetak/streamlink,Dobatymo/livestreamer,beardypig/streamlink,sbstp/streamlink,chrippa/livestreamer,melmorabity/streamlink,hmit/livestreamer,chhe/streamlink,gravyboat/streamlink,Saturn/livestreamer,gtmanfred/livestreamer,Masaz-/livestreamer,blxd/livestreamer,fishscene/streamlink,hmit/livestreamer,Masaz-/livestreamer,mmetak/streamlink,gravyboat/streamlink,javiercantero/streamlink,streamlink/streamlink,caorong/livestreamer,back-to/streamlink,wolftankk/livestreamer,charmander/livestreamer,flijloku/livestreamer,okaywit/livestreamer,beardypig/streamlink,chrippa/livestreamer,bastimeyer/streamlink,chhe/streamlink,Klaudit/livestreamer,wolftankk/livestreamer,intact/livestreamer,blxd/livestreamer,chhe/livestreamer,sbstp/streamlink,jtsymon/livestreamer,fishscene/streamlink,Saturn/livestreamer,back-to/streamlink,gtmanfred/livestreamer