Fix regression

[tkan145]

What

Discovered this regression while investigate THREESCALE-11741

In PR #1503 we return nil and error in case of an error, however some code paths do not handle the error but instead check the error field of the response object and lead to unexpected behavior (bypass backend-listener authrep call).

The return value of http_ng backends is also inconsistent, some backends return errors while others return response objects with error fields. This leads to some code paths checking for errors (always nil for resty backends) while others check the response object for errors or ignore errors altogether. This needs to be investigated

Verification steps

• Checkout this branch
• Build a new runtime image

make runtime-image IMAGE_NAME=apicast-test

• Get into dev-env

dev-environments/plain-http-upstream

• Edit docker-compose.yml as follow

diff --git a/dev-environments/plain-http-upstream/docker-compose.yml b/dev-environments/plain-http-upstream/docker-compose.yml
index ebf84ebc..182a6294 100644
--- a/dev-environments/plain-http-upstream/docker-compose.yml
+++ b/dev-environments/plain-http-upstream/docker-compose.yml
@@ -15,6 +15,7 @@ services:
       APICAST_WORKERS: 1
       APICAST_LOG_LEVEL: debug
       APICAST_CONFIGURATION_CACHE: "0"
+      BACKEND_ENDPOINT_OVERRIDE: "http://invalid-backend"
     expose:
       - "8080"
       - "8090"

• Start gateway

make gateway IMAGE_NAME=apicast-test

• Send request

curl --resolve get.example.com:8080:127.0.0.1 -v "http://get.example.com:8080/?user_key=123"

You should get HTTP/1.1 403 Forbidden