fix: if we hit a 404 in starlette, raise instead of render a 404

[maartenbreddels]

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 the handler after all the imports are done to avoid a circular import
    add_exception_handler()
    solara.Text("hello world")

[render]

Your Render PR Server URL is https://solara-stable-pr-670.onrender.com.

Follow its progress at https://dashboard.render.com/web/srv-cpgsbomv3ddc73ecps8g.

[iisakkirotko]

Looks great! Should be super useful for everyone using Solara, including us :).