V2: Can I fail to status code 503 instead of 502?

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It’s something that nginx offers, in the form of:

Furthermore, it is possible to change the response code to another using the “ = response ” syntax, for example:

`error_page 404 =200 /empty.gif;

If that syntax worked with Caddy, I would expect something like this to work:

server.example.com {
    reverse_proxy * 192.168.0.13:1234
    error_page 502 =503 maintenance.html
}

or perhaps

server.example.com {
    reverse_proxy * 192.168.0.13:1234 {
        error_page 502 =503 maintentance.html
    }
}

Or some other equivalent syntax. I just want Caddy configured to never return 502 for this server, only 503.