Keep audio file info when file is missing

[robbevp]

When a file is missing during a rescan, we currently delete the audio_file instance and add a warning to the rescan log.

The information about an audio file (mostly: location and filename) can still be relevant after a file goes missing, for example to figure out what happened to a file.
Once the rescan has run again, the mod+ has no way of figuring out what the filename and location was.

Reproduction steps

1. Add track and audio file
2. Remove audio file from system
3. Run rescan twice
4. Try and figure out where the track's audio file was located

Expected behaviour

We should keep this info so users can debug missing file easily. It should be marked as 'missing' is some way.
Only once the track is deleted or merged should the audio_file instance be deleted