Check if net error is a proxy connection error

isproxyerror.go

import (
	"errors"
	"net"
)

func IsProxyError(err error) bool {
	var operr *net.OpError
	if errors.As(err, &operr) {
		if operr.Op == "proxyconnect" {
			return true
		}
	}
	return false
}