IDefinedAggregator can't handle error codes from downstream requests

[jasongm86]

Expected Behavior / New Feature

I'm using a custom aggregator and have authentication on the downstream requests. I'm struggling to find a way to handle authentication errors because the IDefinedAggregator takes a list of type DownstreamResponse which in the case where the downstream request is unauthenticated, these responses will be null and this isn't handled before .Aggregate() is called on the custom defined aggregator (for good reason I suppose).

Actual Behavior / Motivation for New Feature

I'd either expect to be able to handle the Ocelot error codes from the DownstreamContext in the custom aggregator or in the event that one or more of the downstream requests is unauthenticated, that UserDefinedResponseAggregator handles that before passing the responses to the aggregator. The former would obviously be better but I'm looking for opinions because it looks to me like anyone using a custom aggregator with authentication should have run into this problem, although I can't see any previous issues on GitHub. Happy to contribute and implement if I get some feedback and validation so raising this issue in the first instance.

Steps to Reproduce the Problem

{
  "ReRoutes": [
    {
      "DownstreamPathTemplate": "whoami/firstname",
      "UpstreamPathTemplate": "/firstname",
      "UpstreamHttpMethod": [
        "Get"
      ],
      "DownstreamScheme": "http",
      "DownstreamHostAndPorts": [
        {
          "Host": "someservice",
          "Port": 80
        }
      ],
      "Key": "FirstName",
      "AuthenticationOptions": {
        "AuthenticationProviderKey": "JwtBearer"
      }
    },
    {
      "DownstreamPathTemplate": "whoami/lastname",
      "UpstreamPathTemplate": "/lastname",
      "UpstreamHttpMethod": [
        "Get"
      ],
      "DownstreamScheme": "http",
      "DownstreamHostAndPorts": [
        {
          "Host": "someservice",
          "Port": 80
        }
      ],
      "Key": "LastName",
      "AuthenticationOptions": {
        "AuthenticationProviderKey": "JwtBearer"
      }
    }
  ],
  "Aggregates": [
    {
      "ReRouteKeys": [
        "FirstName",
        "LastName"
      ],
      "UpstreamPathTemplate": "/whoami",
      "Aggregator": "MyDefinedAggregator"
    }
  ],
  "GlobalConfiguration": {
    "RequestIdKey": "OcRequestId"
  }
}```