Track 502 error to show failed callback due to timeout

When developing locally, the user may not be aware of any timeout issue in the callbacks. Only when the app is deployed, the app might face the gunicorn worker timeout(usually 30s by default).

However, when running into this issue, there is no feedback to the user except in the server log or network log.

When a callback is triggered, dash renderer makes a request to /_dash-update-components with the inputs and states. Then dash takes those and passes them to the callback function and sends the return value back to dash renderer.
Dash Renderer doesn't handle the 502 error so the user is unaware that the callback failed.

It will be very useful to allow the user to catch this 502 error in the dash app and avoid the timeout(or at least identify this issue easily)

Start by tracing the dash renderer request to /_dash-update-components and the callback response path. Define and test behavior for a 502 caused by a callback timeout, with a user-visible way to identify the failed callback; the issue names no files or tests, so repository exploration is required.