crypto/tls: add QUICEvent.Err

// This program demonstrates how QUICEvent.Err would be used once the change
// ships. It does not actually create a QUIC connection because that requires
// network I/O, but it shows the structure and how an error would be checked.
package main

import (
	"fmt"
)

type QUICEvent struct {
	// Set for QUICNoEvent
	Err error
	// existing fields as-is
}

const (
	// QUICNoEvent indicates that there are no events available.
	// QUICEvent.Err is set if the connection has encountered a fatal error.
	QUICNoEvent = iota
)

func main() {
	// Simulate receiving an event with an error.
	event := QUICEvent{Err: fmt.Errorf("handshake failed")}
	if event.Err != nil {
		fmt.Println("Fatal error detected:", event.Err)
	} else {
		fmt.Println("No error")
	}
}