Turn error into panic when port is used

---

examples/panic-http/panic_http.go

package main

import (
    "fmt"
    "net/http"
)

func mainPage(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "Hello World")
}

func main() {
    http.HandleFunc("/", mainPage)
    fmt.Println("Going to listen on http://localhost:5000  Ctr-c to stop the server.")
    err := http.ListenAndServe("127.0.0.1:5000", nil)
    if err != nil {
        //log.Fatal(err)
        panic(err)
    }
}