Only formats that are available should be selectable

[probonopd]

Only formats that are available should be selectable in the GUI. Currently you can select video formats that a particular video may or may not be available in, resulting in

[youtube] YAtS6XEwyQY: Downloading webpage
[youtube] YAtS6XEwyQY: Downloading video info webpage
[youtube] Downloading just video YAtS6XEwyQY because of --no-playlist
ERROR: requested format not available

[JannikHv]

I assumed this would happen quite often. The "right" way to do it would be to list available combinations (format / quality) und let the user choose one. This would sadly influence the simple usability.