Ensure raw error for API exceptions by marking NiceGUI page requests in request scope

[evnchn]

Motivation

Fix #5508, where NiceGUI returns the HTML for the "sad face error page" under this FastAPI GET request:

@app.get("/api_error")
def api():
    return {"response": 1/0}

Problems:

• Wasteful in bandwidth
• Difficult to parse and detect (remember: client_id is dynamic)
• Non API-like behaviour

Optimally for @app, it should act as much like FastAPI as possible. This PR does exactly that.

Implementation

• Mark NiceGUI page requests in request scope
• Only return client.build_response(request, 500) if found
• If not we raise exception and FastAPI proceeds to "Internal Server Error" exactly like if a plain FastAPI app would.

Results

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests have been added (058d488).
• Documentation is not necessary (I consider this a bug, frankly).

[falkoschindler]

Thanks, @evnchn!
Looks good to me. 😊

[falkoschindler]

Sorry, I approved too early. I found an issue while the CI was running:

We are logging twice for raw API requests.

• Should we move log.exception(exception) below the early exit? Otherwise we log twice for exceptions in raw API endpoints.

• Alternatively we could return a response rather than raising:

  return PlainTextResponse('Internal Server Error', status_code=500)

  This way the _exception_handler_500 always logs and - depending on the scope - returns a plain text or page response, without raising and letting Starlette handle the problem intransparently.

What do you think?

[evnchn]

This should fix things. We do not mock the Internal Server Error because one may customize the FastAPI error handling.

[evnchn]

And if I am not mistaken, the native FastAPI behavior is to not log. Not nice to the developer, but again, it's not called NiceAPI.