Missing response header X-Amz-Function-Error

I wasn't sure whether to open this here or over in https://github.com/aws/aws-lambda-nodejs-runtime-interface-client

With a simple example that always throws an error:

```Dockerfile
FROM public.ecr.aws/lambda/nodejs:12
RUN echo 'module.exports = { handler: () => { throw new Error("failure"); } }' > index.js
CMD ["index.handler"]
```

When I hit the endpoint:
```
$ curl -qv http://localhost:8080/2015-03-31/functions/function/invocations -d '{}'
* Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> POST /2015-03-31/functions/function/invocations HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.64.1
> Accept: */*
> Content-Length: 2
> Content-Type: application/x-www-form-urlencoded
>
* upload completely sent off: 2 out of 2 bytes
< HTTP/1.1 200 OK
< Date: Mon, 18 Jan 2021 23:27:37 GMT
< Content-Length: 184
< Content-Type: text/plain; charset=utf-8
<
* Connection #0 to host localhost left intact
{"errorType":"Error","errorMessage":"failure","trace":["Error: failure"," at Runtime.handler (/var/task/index.js:1:43)"," at Runtime.handleOnce (/var/runtime/Runtime.js:66:25)"]}* Closing connection 0
$
```

As expected, the response payload has the shape described in https://docs.aws.amazon.com/lambda/latest/dg/nodejs-exceptions.html and the status is 200 to denote a function error.

However, shouldn't the response headers include `X-Amz-Function-Error` so that the error status can be determined without relying on heuristics?

Also, `Content-Type` should be `application/json`.

Bắt đầu bằng việc tái hiện với Dockerfile và yêu cầu curl tới endpoint invocation cục bộ. Xác minh các response header và payload hiện tại, sau đó lần theo cách emulator xử lý response; hoàn tất khi các lỗi hàm bao gồm X-Amz-Function-Error và các response sử dụng application/json.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.