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

Implementations shall use exceptions instead.

Use this function instead of CheckError() when we already know an
error has occurred.

Obsoleted by commit d4d4d621

Don't need to clear the caller-specified one.

Necessary to preserve type information.  The try/catch sequence didn't
work previously.

Same fix as in commit 1c904000

For exception-safety.