Hotloading fix after rP25210
Emplace does not replace existing element, insert_or_assign does.
While at it:
- Clean up the 'outdated' logic and reuse it for actors
- When an actor fails to load, return a placeholder. This improves hotloading of broken actors, and makes Cunit behaviour more predictable.
- Some minor cleanup
Reported by: Stan
Fixes #6157
Differential Revision: https://code.wildfiregames.com/D3882