fsolve() accepts arbitrarily complex functions. You cannot numerically calculate the truncation error of arbitrarily complex functions.
For example, suppose the function calculates the number of Odd Perfect Numbers, and suppose the solution come out with is 0. The truncation error would be the actual number of odd Perfect Numbers, a value that no one knows even though it has been actively studied for nearly 400 years. In order for fsolve() to return a (valid) truncation error, fsolve() would have to be able to solve a major outstanding problem in mathematical theory. No Can Do.
