NTC based voltage divider relative error
The maximum error in the voltage output of a divider is not the sum of the errors, as your link shows. It only approaches the sum if one resistor is much bigger than the other. 

The maximum error will occur at one extreme or the other for each resistor and for the thermistor. For N parts, there are only 2^N combinations, so it's possible to do an exhaustive search numerically at each data point without any thinking, but it should be obvious which way the parts will affect the output. 

If your only requirement is to calculate the maximum error, simply assume each part is at its worst-case tolerance limit for the reading to be high or low, and calculate said error.