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 branch 'layering-fix-slash-nerf' of https://github.com/kevingranade/Cataclysm-DDA into Bugfixes
Merge branch 'fix-vending-special-place' of https://github.com/BevapDin/Cataclysm-DDA into Bugfixes