Fix translation updater script: Handle nested modpacks

[appgurueu]

Trivial fix for #14281, #14282: update_mod -> update_folder; modpacks can contain other modpacks. Also added a warning if what's supposed to be a mod folder is missing init.lua.

How to test

See the issues.

[SmallJoker]

I think it would make sense to also treat directories named "mods" where ../game.conf is present as modpacks. That would allow proper translation updates of games (in respect to my other code comment).

[appgurueu]

That would allow proper translation updates of games (in respect to my other code comment).

With the current state of this PR, ./mod_translation_updater.py -r path/to/game/mods works as expected. Would you like me to make ./mod_translation_updater.py -r path/to/game work too?

[SmallJoker]

Would you like me to make ./mod_translation_updater.py -r path/to/game work too?

Yes, I think for modders it's reasonable to assume that this would work too, hence I'd definitely support such functionality.

[appgurueu]

Would you like me to make ./mod_translation_updater.py -r path/to/game work too?

Yes, I think for modders it's reasonable to assume that this would work too, hence I'd definitely support such functionality.

Okay, added support for ./mod_translation_updater.py path/to/game (the -r didn't make sense much sense with our current definition of -r - which I don't want to change - there, my bad).

[SmallJoker]

Works on my machine. Thank you for the changes.