Apparently some users have had problems installing NodeCore because
they assumed it was a modpack and just copied the mods/* into
~/.minetest/mods and enabled them in an MTG game world. The result
apparently didn't immediately crash but wasn't really playable
either.
Detect NodeCore being combined with a mod named "default" and
throw an error immediately. This should catch like 90% of the
games out there. Some like LoTT or IKEA don't use a default, but
they're also not the ones that players would naively try to run
arbitrary mods on top of.
