Warn when adding [non-official] mods to an unmodded save

[sparr]

Attempting to load a save with the wrong mods produces a warning and a prompt to load the mods that were used in the save.

However, there is no such prompt if you're just adding a mod. This is usually/mostly fine. I would propose an exception for adding the first mod(s) to a save that was previously not using any mods (other than the official 2.0 mods). It is currently too easy to accidentally leave one or more mods enabled, load a "vanilla" save, and keep playing without unknowingly disabling [Steam] achievements.

[sparr]

Just got bit by this again

[flamefield]

I also encountered it, but fortunately I found the problem within 15 minutes and restored the save through autosave without adding mods