Merge pull request #7665 from Rivet-the-Zombie/even-more-fixes

More price fixes.
Fix float/int rounding bugs that were making layering too forgiving.
load item group for vending machine specials from mapgen json

Fix the error reporting, second parameter to JsonObject::throw_error is the name of the offending member
throw_error does not perform printf-style string formatting.
Merge remote-tracking branch 'CleverRaven/master' into FrankenMod
Merge remote-tracking branch 'kevingranade/layering-fix-slash-nerf' into FrankenMod
Merge remote-tracking branch 'BevapDin/fix-vending-special-place' into FrankenMod