How to compute relative error for the rounding of floating point numbers when the rounded number is 0?
If x = 0 and is rounded to 0 then the error is 0, and the relative error is whatever you want it to be. 

If x ≠ 0 and is rounded to 0, then the absolute error is x, and the relative error is 1 (or 100% if that is more intuitive). 

If x = 0 and is round to y ≠ 0 then the absolute error is y, and the relative error is infinite or undefined, whatever you prefer.