playlist: log errors during loadPlaylist()

Don't call command_error() if loading a song from the playlist fails.
This may result in assertion failures, since command_error() may be
called more than once.

src/command.c

@@ -591,7 +591,7 @@ handle_load(struct client *client, G_GNUC_UNUSED int argc, char *argv[])
 {
 	enum playlist_result result;
 
-	result = loadPlaylist(client, argv[1]);
+	result = loadPlaylist(argv[1]);
 	return print_playlist_result(client, result);
 }