improve warning message according to approximation mode

[howaboutuser]

set approximation exact
set angle rad
(cosx+cotx)/(cscx+1)

warning: To avoid division by zero, the following must be true: x ≠ 3.141592654n.
warning: To avoid division by zero, the following must be true: x ≠ 3.141592654n − 1.570796327.
(cos(x radians) + cot(x radians)) / (csc(x radians) + 1) = (2 × cos(x) + sin(2x)) / (2 + 2 × sin(x))

Expect pi and pi/2 instead of 3.141592654 and 1.570796327.