Caller should check the ResponseExtra.HasError() first to see whether the request fails (#28796)

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

modules/private/actions.go

@@ -22,7 +22,7 @@ func GenerateActionsRunnerToken(ctx context.Context, scope string) (string, Resp
 	})
 
 	resp, extra := requestJSONResp(req, &responseText{})
-	if resp == nil {
+	if extra.HasError() {
 		return "", extra
 	}
 	return resp.Text, extra