This allows middleware or error handlers to catch the 404 and do
something custom.

Example:

```
import solara

clicks = solara.reactive(0)

added = False

def add_exception_handler():
    global added
    print("add_exception_handler", added)
    if not added:
        import solara.server.starlette
        from starlette.responses import HTMLResponse
        async def server_error(request, exc):
            return HTMLResponse(content="My custom 404", status_code=exc.status_code)

        solara.server.starlette.app.add_exception_handler(404, server_error)
        added = True

@solara.component
def Page():

    add_exception_handler()
    solara.Text("hello world")
```