How to define a custom error page on AppEngine flex?

Question

Currently if the server is down or an instance is starting AppEngine flex shows an ugly page with

502 Server Error

Error: Server Error

The server encountered a temporary error and could not complete your request.

Please try again in 30 seconds.

Is it possible to provide a custom error page so I can match it the design of the app?

I see app engine standard has error_handlers, it is not in the flex doc. I tried it on flex and it does not work.

Answer 1

As per the answer of @new name:

Nope. To provide a custom 502 error, the server would have to be able to respond. The server can't respond if it is returning an error.

500 errors are tailored by the server if it does not respond to a certain command or function. We can't perform custom errors since it came on the server-side.