Non-loadable classes in config files are silently ignored

When loading a custom config file I discovered, that non-loadable classes are just silently ignored while digging into a bug (HumanCompatibleAI/imitation#664).

The root cause seems to be in

sacred/sacred/serializer.py

Line 33 in f7311fc

return json.decode(_json.dumps(flat), keys=True)

where you should call json.decode with the on_missing parameter set to error or at least warn.

This is a change introduced from jsonpickle 2.1.0 to 2.2.0.