How to calculate measurement error in a voltage divider?
You can calculate the error using partial derivatives, if you write the formula Vout=f(Vref,R1,R2) and calculate its partial derivatives. This would give you error with regard to:

Voltage error
R1 resistor error
R2 resistor error

The ADC quantization can't be evaluated in this calculation, however it can be added latter, for example:
E =+2% / -0.5% +/- 2LSB