Fix script error alert (#37458)

After using CSP nonce, the "onerror" doesn't work anymore. Change it to
use a global variable to detect

Also help users like #37379 to catch errors more easily.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/js/vitest.setup.ts

@@ -12,6 +12,7 @@ window.config = {
   enableTimeTracking: true,
   mermaidMaxSourceCharacters: 5000,
   i18n: {},
+  frontendInited: false,
 };
 
 window.testModules = {};