videotoolbox: slightly improved error handling
videotoolbox: slightly improved error handling

Error 8973 is triggered if the number of open HW decoding sessions is > than the number of available HW decoders

Further, a failing session creation should not invalidate the entire video decoding chain but just trigger the fallback

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc