If the base class is not accessible, the "catching" the base class
won't work.  This caused the fatal error:

 terminate called after throwing an instance of 'LibmpdclientError'

SimpleDatabase::Visit() always throws exception when visiting a song.
Adding a song to the queue always results in a "No such directory"
error response, although the song is successfully added.  This
behaviour was introduced by commit
3ff728ab.  This patch fixes this.

https://bugs.musicpd.org/view.php?id=4601

Speed up container_scan() again, by eliminating the need to call
scan_file() for each item.

Implementations shall use exceptions instead.