db/simple/Save: migrate from class Error to C++ exceptions

src/SongSave.hxx

@@ -38,11 +38,9 @@ song_save(BufferedOutputStream &os, const DetachedSong &song);
  * Loads a song from the input file.  Reading stops after the
  * "song_end" line.
  *
- * @param error location to store the error occurring
- * @return true on success, false on error
+ * Throws #std::runtime_error on error.
  */
 DetachedSong *
-song_load(TextFile &file, const char *uri,
-	  Error &error);
+song_load(TextFile &file, const char *uri);
 
 #endif