Fixes a deadlock caused by commit
31ab78ae.  That commit was not
actually bad - just these two calls have always been bad, which went
unnoticed for a long time.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Fixes aborted "load" commands due to SongLoader failure.

Obsolete class CloseSongEnumerator, which was a kludge.