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/key.go

@@ -27,7 +27,7 @@ func AuthorizedPublicKeyByContent(ctx context.Context, content string) (string,
 	req := newInternalRequest(ctx, reqURL, "POST")
 	req.Param("content", content)
 	resp, extra := requestJSONResp(req, &responseText{})
-	if resp == nil {
+	if extra.HasError() {
 		return "", extra
 	}
 	return resp.Text, extra