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.

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

Replaces StatFile(), with a portable data object.

PathExists() should better do what CheckAccess() does, and
CheckAccess() doesn't do what its name implies.

The old struct config_param remains only for top-level string options.

Return false on error, not nullptr.