Notify user if their primary media locations are not mounted

[qq99]

It should be possible to detect that the primary drives are not mounted.

For example, when trying to discover if something needs a transcode, and the backing file store is not mounted, it will return "" and assume that, yes, it does need a transcode.

When trying to play media, it will just silently fail with no playback.

Media cleaning actions could be dangerous -- File.exist? will return false for specific raw file paths