revising error message to include version number

Signed-off-by: Christina Wen <christina@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -648,8 +648,8 @@ func (c *Conn) Ping(ip netaddr.IP, cb func(*ipnstate.PingResult)) {
 	}
 
 	dk, ok := c.discoOfNode[peer.Key]
-	if !ok {
-		res.Err = "no discovery key for peer (old Tailscale version?). Try: ping 100.x.y.z"
+	if !ok { // peer is using outdated Tailscale version (pre-0.100)
+		res.Err = "no discovery key for peer (pre Tailscale 0.100 version?). Try: ping 100.x.y.z"
 		cb(res)
 		return
 	}