Spooky: http.Error(w, [text], [statusCode]) responds the code without the text

Hi All!
I am stuck with this bizarre phenomenon that the http.Error function does not repond with the customizable text defined by the second argument.
Suppose a simple ill http server with this function that responds an error as always:
------------------------------
func (con *Net) ServeHTTP(w http.ResponseWriter, r *http.Request) {
  http.Error(w, "Whatever! I return an error", 403)
  return
  //the original functional codes are below and they won't get executed now
  ......
}
==================
So it is easy to transform any of your functional go http code with the two magic lines and test the functionality of http.Error.

And here is some javascript that visits a particular url where your ServeHTTP serves with GET method:
-----------------------------------
async function get(url){
    let re
    try{
        re=await fetch(url,{method: 'GET'})
    }catch(err){
        alert(''The url your provided is not reachable!")
        return Promise.reject(1)
    }
  if (re.status != 200){  
    alert(`Server responded with an error：${re.status} ${re.statusText}`)
    console.log(`Server responded with an error：${re.status} ${re.statusText}`)
    return Promise.reject(re.status)
   }else{ // irrelevant in this situation
    return re.json()
  }
}

function test_Error403(){
  get('[where the golang ServeHTTP function serves]').
  then((msg) => console.log(msg))}
=============================
Then you can link test_Error403() function with a button on an html file, but be aware to change '[where the golang ServeHTTP function serves]' to real url.

The bizarre situation is I always get alert or console log like:
  Server responded with an error：403
while what I expect is:
  Server responded with an error：403 Whatever! I return an error

So is there anything wrong with the syntax? How can I respond the client with an arbitrary string as `statusText` ?

Thanks in advance!