Non-existant files should be marked as invalid in playlist (to not cause infinite loop in repeat mode)

When a non-existant file (or file that is otherwise unplayable) is part of a playlist, each time that file is reached (say, if repeat mode is on), VLC will output an error.

More importantly, if a non-existant file is opened so that it is the only item in the playlist, VLC enters an infinite loop where it keeps trying to open the file and failing, reporting the error indefinitely in the message log, and making the interface unresponsive.

Instead the offending non-existant file should be removed from or marked as invalid in the playlist, so that VLC just skips over it, and gives up.