Use correct has error check for internal responses (#28796) (#28798)

Backport #28796 by @wxiaoguang

`resp != nil` doesn't mean the request really succeeded. Add a comment
for requestJSONResp to clarify the behavior.

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/private/mail.go

@@ -30,7 +30,7 @@ func SendEmail(ctx context.Context, subject, message string, to []string) (strin
 	})
 
 	resp, extra := requestJSONResp(req, &responseText{})
-	if resp == nil {
+	if extra.HasError() {
 		return "", extra
 	}
 	return resp.Text, extra