Add back window.onerror handler

[joshdover]

In #41663, the global window.onerror error handler was not being imported / registered, so we've been allowing some silent errors for a few weeks now.

Unfortunately, adding it back is causing Kibana to crash due to #22913. Once that issue is resolved, we should add this error handler back, directly in the New Platform's FatalErrorService.

[elasticmachine]

Pinging @elastic/kibana-platform

[joshdover]

Timebox 1 hour investigation, triage next week

[joshdover]

We have two things now triggering window.onerror:

• ResizeObserver loop limit exceeded (ResizeObserver loop limit exceeded #22913)
• Error when loading EuiCodeEditor due to missing ace theme, shows as a SyntaxError when trying to parse a HTML error page.

Both of these are fixable, but probably need to happen in EUI.

I looked more into the behavior of window.onerror and I'm now of the opinion we should not have a global handler that crashes the frontend, because:

• Any uncaught JavaScript error can cause this error to fire, including errors triggered by 3rd party browser extensions.
• Not all uncaught errors invalidate the state of Kibana, such as the ResizeObserver one above. Crashing the entire frontend seems heavy handed and even showing a error toast about something the user can't take action on is just unhelpful noise.

Options going forward:

1. Fix these issues above and add back the window.onerror handler.
   • This could result in crashing Kibana unnecessarily
2. Add a window.onerror handler that only shows a toast rather than crashing the entire frontend
   • Will result in some ugly UX until the above issues are fixed (though more can arise) however it's much less heavy-handed than the previous behavior
3. Once the frontend logging service is built & can forward logs to the server, we could add a window.onerror handler that simply captures this information for debugging purposes.
4. Don't add a window.onerror handler at all
   • Would probably be ok and shouldn't make support harder since these errors still appear in the Web Console.

[joshdover]

5. Use telemetry or some sort of error collection to report back to us.