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/index.ts

@@ -171,3 +171,5 @@ const initDur = performance.now() - initStartTime;
 if (initDur > 500) {
   console.error(`slow init functions took ${initDur.toFixed(3)}ms`);
 }
+
+window.config.frontendInited = true;