improve itdb_parse error message on empty iPods
improve itdb_parse error message on empty iPods

The error message returned when trying to call itdb_parse on a mount
where neither an iTunesDB or iTunesCDB could be found was referring to
a "(null)" filename that couldn't be found. Changes the error message
to something more sane.

hyperair/libgpod,neuschaefer/libgpod,neuschaefer/libgpod,hyperair/libgpod,hyperair/libgpod,neuschaefer/libgpod,hyperair/libgpod,hyperair/libgpod,neuschaefer/libgpod,neuschaefer/libgpod