How to reduce op amp output error?
Im curious why you are using aplification. In my opinion simple cell voltage measuerement could look just like this (i drawed it only for 3 cells):



This should give you acurracy probably below 0.5% + resistor tolerance, but you can compensate resistor tolerance in software.



This is how very acurrate cell measurement can be done

This is part of cell voltage measurement circuit from well designed 6 cell battery charger:



You can find full version of this schematic here

LM324 amplifiers were used in very good Turnigy Accucell 6 and some other RC battery chargers in cell balancer circuits.