Ensure custom errors are served for GOV.UK Chat

We've added custom error pages for our 500 and 503 errors. This updates
the vcl_error subroutine to serve these custom error pages when the
'Govuk-Rendered-Error' header is set.

Our application returns a 503 status code when public access is toggled
off. I'm unsure if there are other cases where we may receive a 503
status code and Roch thinks that the load balancer will return a 500 status
if the application is down.

To ensure that we don't accidentally move to the deliver subroutine when
a 500 or 503 is returned without our custom error page we came up with the
idea of adding a header which indicates that we want to serve the page
from the application.

This change adds a check for the 'Govuk-Rendered-Error' header in the
vcl_error subroutine. If present it moves to the deliver subroutine
to serve the custom error page.

www/www.vcl.tftpl

@@ -230,7 +230,7 @@ sub vcl_recv {
   if (req.method !~ "^(GET|HEAD|POST|PUT|DELETE|OPTIONS|PATCH|FASTLYPURGE)") {
     error 806 "Not Implemented";
   }
-  
+
   %{ if private_extra_vcl_recv != "" ~}
     ${private_extra_vcl_recv}
   %{ endif ~}
@@ -676,6 +676,10 @@ sub vcl_error {
     }
   }
 
+  if (beresp.http.Govuk-Rendered-Error) {
+    return (deliver);
+  }
+
   # Assume we've hit vcl_error() because the backend is unavailable
   # for the first two retries. By restarting, vcl_recv() will try
   # serving from stale before failing over to the mirrors.