Allow respond in the response path

[ceastman-ibm]

Title: One line description

Description:

Describe the the desired behavior, what scenario it enables and how it
would be used.

A microservice in the istio service mesh causes a 4xx or 5xx error, instead of just returning the status code to the browser. We have a microservice that renders a nice custom error page. We want to modify the response body to be this nicely rendered custom error page.

nginx has this functionaly, it would be great if envoy had it also.

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Any extra documentation required to understand the issue.
here is the snippet of lua code inside the envoy_on_response function that I am trying to use:

        -- This section of code is for redirecting to maintenance if a 4xx or 5xx status code is detected
          if headerStatus ~= nil then
            if string.match(headerStatus, '[4-5].*') and headerStatus ~= "401" then
              response_headers, response_body = response_handle:httpCall(
                "outbound|80||maintenance.ondeck.svc.cluster.local",
                {
                  [":method"]   = "GET",
                  [":path"]     = "/maintenance/"..headerStatus,
                  [":authority"] = "maintenance.ondeck.svc.cluster.local"
                },
                "",
                1000
              )
              response_handle:respond(
                {[":status"] = "200",
                 ["envoy-fallback"] = "true"},
                response_body)

            end
          end

i get the following error:
[istio-ingressgateway-78bcc4574c-9pwmx] 2020-09-03T19:21:21.223613Z error envoy lua [external/envoy/source/extensions/filters/http/lua/lua_filter.cc:685] script log: [string "function encodeChar(chr)..."]:112: respond not currently supported in the response path

[linsun]

@ceastman-ibm is this same issue as #5998 or different?

[ceastman-ibm]

@linsun looks to be the same issue, ill close this one.