[v1] Exceptions inside `before_main` are silent

[oussama-gourari]

When an exception occurs inside the before_main callback, it is not reported anywhere, and the app will be stuck in the "Working..." state. Example:

import flet as ft


def before_main(page):
    1 / 0


def main(page):
    page.add(ft.Button("Button"))


ft.run(main=main, before_main=before_main)