Until now, using --sub-file would add only subtitle tracks from the
given file. (E.g. if you passed a video file, only the subtitle tracks
from it were added, not the video or audio tracks.)
This is slightly messy (because streams are hidden), and users don't
even want it, as shown by #5132. Change it to always add all streams.
But if there's no stream of the wanted type, we still report an error
and do not add any streams. It's also made sure none of the other track
types are autoselected.
Also adjust the error messages on load failure slightly.
Fixes#5132.
80d43ee4e6