Detailed error message for generic type parameter of find_contours_with_threshold (#694)

[sakird]

Below code does not help developers in case of an error with the size set using generic type parameter:

contour_points.push(Point::new(cast(pos3.x).unwrap(), cast(pos3.y).unwrap()));

To help developers understand the error easily, code is updated with the error message like below:

contour_points.push(Point::new( cast(pos3.x).expect("Generic Type Parameter T should be bigger than width."), cast(pos3.y).expect("Generic Type Parameter T should be bigger than height."), ));

[cospectrum]

fixes https://github.com/image-rs/imageproc/issues/694