Replace uses of os.IsExist and os.IsNotExist with errors.Is.

The former don't properly unwrap errors.

rtpconn/webclient.go

@@ -1299,7 +1299,7 @@ func leaveGroup(c *webClient) {
 
 func closeDownConn(c *webClient, id string, message string) error {
 	err := delDownConn(c, id)
-	if err != nil && !os.IsNotExist(err) {
+	if err != nil && !errors.Is(err, os.ErrNotExist) {
 		log.Printf("Close down connection: %v", err)
 	}
 	err = c.write(clientMessage{
@@ -1413,7 +1413,7 @@ func handleClientMessage(c *webClient, m clientMessage) error {
 		if err != nil {
 			var e, s string
 			var autherr *group.NotAuthorisedError
-			if os.IsNotExist(err) {
+			if errors.Is(err, os.ErrNotExist) {
 				s = "group does not exist"
 			} else if errors.Is(err, group.ErrAnonymousNotAuthorised) {
 				s = "please choose a username"