Panics during startup of web-viewer are not reported to the user

[emilk]

If the web-viewer panics, we should show that as a nice message to our users (in a div). We have code for that in our index.html, but it doesn't work if the crash happens during startup (creating of WebHandle) as in #2032.

This is because the "Have we paniced?" check is installed in on_app_started.

This requires some improvements to eframe (on it)

[emilk]

Will be fixed by:

• eframe web: detect and report panics during startup emilk/egui#2992