Unexpected empty response when lambda function's process terminates

The RIE behaves differently from the "production" AWS Lambda environment when a Lambda function's process terminates.

Concretely, the production environment responds with a message that contains the process's exit status, e.g.:

```json
{
"errorType": "Runtime.ExitError",
"errorMessage": "RequestId: 7f399cc8-8841-fefa-0af7-ba803f3f21e3 Error: Runtime exited with error: exit status 42"
}
```

The Lambda RIE however does not send such a response. Instead, when the Lambda function's process terminates, the RIE only responds with an _empty_ response body, which does not resemble the behavior of the production environment.