Proof of concept

main.go

package main

import (
	"fmt"
	"net/http"

	"github.com/gocolly/colly"
)

type MyCollector struct {
	*colly.Collector
}

func NewMyCollector(collector *colly.Collector) *MyCollector {
	if collector == nil {
		collector = colly.NewCollector()
	}
	c := MyCollector{
		Collector: collector.Clone(),
	}
	return &c
}

func main() {

	URL := "http://www.gmail.com/"

	c := NewMyCollector(nil)

	c.ParseHTTPErrorResponse = true
	c.RedirectHandler = func(req *http.Request, via []*http.Request) error {
		return http.ErrUseLastResponse
	}

	c.OnResponse(func(r *colly.Response) {
		if r.StatusCode == 200 {
			panic(fmt.Errorf("expected 301, got %d - %s", r.StatusCode, r.Request.URL.String()))
		}
	})
	c.OnError(func(r *colly.Response, err error) {
		fmt.Println("error", err, r.StatusCode, r.Request.URL.String())
	})

	c.Visit(URL)
}