Fixes aborted "load" commands due to SongLoader failure.

For std::runtime_error.  See http://bugs.musicpd.org/view.php?id=4509

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

Obsoleted by commit d4d4d621

release v0.19.14

Fixes regression by commit 42f7df96

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

This reduces overhead for two reasons:

1. calls to Remove() are non-blocking

2. RunDeferred() may work on large chunks at a time, reducing the
   number of RunDeferred() calls

There's no point in passing a LightSong reference here; the callee is
interested only in the URI.