net: ErrClosed should implement Error

[josharian]

package main

import (
	"fmt"
	"net"
)

func main() {
	err := net.ErrClosed
	_, ok := err.(net.Error)
	fmt.Println(ok)
}

This prints false. I think it should print true, and that both Temporary and Timeout should return false.

[josharian]

cc @bradfitz @ianlancetaylor for a decision

[ianlancetaylor]

SGTM. Sent a CL.

[gopherbot]

Change https://golang.org/cl/307030 mentions this issue: net: make ErrClosed and ParseError implement net.Error