When stock data cannot be loaded, the widget now shows a "Stock data is not available." message instead of sitting on loading placeholders. It retries once in the background about a minute later and recovers on its own if that attempt succeeds. Showing the error records a widgets_error telemetry event (error_type "load_error"), and the message text is a new localizable string. Differential Revision: https://phabricator.services.mozilla.com/D311484