net: Make errClosing public

[erikdubbelboer]

I would like to propose to make net.errClosing public.

One use case for having this error exposed is to detect http client timeouts as demonstrated by this gist.

At the moment the only way to detect a timeout is to compare error strings:

if res, err := client.Get("http://127.0.0.1:12345/"); err != nil {
  if strings.HasSuffix(err.Error(), "use of closed network connection") { 

If the error string would ever change in the future this detection would break.

[minux]

Dup of #4373.