Mathematica: MathLink error messages

I think I am starting to understand how to link functions written in C/C++ to Mathematica. The problem I'm facing is that I don't know how to send error messages from my C wrapper to Mathematica. After searching in google I found this MathLink Tutorial.

Section 1.7 gave me an insight as to how to send error messages but I am getting weird results. Here is the code I am working with.

//File cppFunctions.h
#ifndef CPPFUNCTIONS_H
#define CPPFUNCTIONS_H
class Point {
public:
    double x, y;
    Point(){ x=y=0.0;}
    Point(double a, double b): x(a), y(b) {}
};
class Line {
public:
    Point p1, p2;
    Line(void) {}
    Line(const Point &P, const Point &Q): p1(P), p2(Q) {}
    double distanceTo(const Line &M, const double &EPS = 0.000001){
        double x21 = p2.x - p1.x;     double y21 = p2.y - p1.y;
        double x43 = M.p2.x - M.p1.x; double y43 = M.p2.y - M.p1.y;
        double x13 = p1.x - M.p1.x;   double y13 = p1.y - M.p1.y;
        double den = y43*x21 - x43*y21;
        if (den*den < EPS) return -INFINITY;
        double numL = (x43*y13 - y43*x13)/den;
        double numM = (x21*y13 - y21*x13)/den;
        if (numM < 0 || numM > 1) return -INFINITY;
        return numL;
    }
};
#endif

The file cppFunctions.h declares the classes Point and Line. I have stripped this class to the bare minium except for the function that I want to use in Mathematica. I want to find the distance from one line to another. This function is the C version of lineInt in wireframes in Mathematica. To use this function in Mathematica we need a wrapper function that obtains the input from Mathematica and sends the output back to Mathematica