Valid call returns 500 without neither logging nor validation error

With openapi-cop 5cb4e9f988cc7150374db3b3dad383281eaf7c8d running in Docker, I get a `500` back from the proxy but no indication of any errors.

I would expect one of:
1. The original (valid) `404` response
1. An explanation of what's wrong, both on the openapi-cop console and in the `Openapi-Cop-Validation-Result` header I got back

# Sequence Diagram

```mermaid
sequenceDiagram
Test Driver->>openapi-cop: DELETE /internal/test/plans
openapi-cop->>App Under Test: DELETE /internal/test/plans
App Under Test->>openapi-cop: 404 (documented response)
openapi-cop->>Test Driver: 500
```

# Log Output

Console output, note the `"valid": true` line.

```
pfcapi-openapi-cop-1 | 2022-12-13T12:33:40.431Z openapi-cop:proxy Received server response with status code 404
pfcapi-openapi-cop-1 | 2022-12-13T12:33:40.432Z openapi-cop:proxy Validation results [DELETE /internal/test/plans] {
pfcapi-openapi-cop-1 | "request": {
pfcapi-openapi-cop-1 | "valid": true,
pfcapi-openapi-cop-1 | "errors": null
pfcapi-openapi-cop-1 | }
pfcapi-openapi-cop-1 | }
```

# Received by Test Driver

Note the combination of `500` and `\"valid\":true`.

```
HTTP/1.1 500 Internal Server Error
Connection: close
Date: Tue, 13 Dec 2022 12:33:40 GMT
Openapi-Cop-Source-Request: {\"method\":\"DELETE\",\"path\":\"/internal/test/plans\",\"headers\":{\"host\":\"subscriptions:5048\",\"user-agent\":\"Go-http-client/1.1\",\"authorization\":\"Basic ***\",\"x-request-id\":\"***\",\"x-span-parent-id\":\"***\",\"accept-encoding\":\"gzip\",\"connection\":\"close\"},\"query\":{}}
Openapi-Cop-Validation-Result: {\"request\":{\"valid\":true,\"errors\":null}}
X-Powered-By: Express
Content-Length: 0
```

## `Openapi-Cop-Source-Request`

Expanded from ^

```json
{
"method": "DELETE",
"path": "/internal/test/plans",
"headers": {
"host": "subscriptions:5048",
"user-agent": "Go-http-client/1.1",
"authorization": "Basic ***",
"x-request-id": "***",
"x-span-parent-id": "***",
"accept-encoding": "gzip",
"connection": "close"
},
"query": {}
}
```