Prepare full C++ exception support in the player thread.

Previously the id was being returned only when a position wasn't
specified.

playlist_file_print() already handles stored playlists.  spl_print()
is never called with a successful result.

Prepare for moving idle events to class Partition.  Right now, it's
just a wrapper for idle_add().

playlist_domain is no longer used for class Error.

This breaks on some standard library implementations.

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