Failure must be indicated by returning false.

Fixes regression by commit 42f7df96

http://bugs.musicpd.org/view.php?id=4470

This is necessary to avoid an assertion failure in UpdateQueuedSong()
after LockSeek() has failed.

Fixes disappearing duration of remote songs during playback.

See http://bugs.musicpd.org/view.php?id=4492

Regression by commit 738583e3

A priority should be a volatile thing: it should schedule a song for
playing back once, but it should not affect the next time the queue
gets played.

Empty currently, but code will be added song.