Error in REST audit layer produces misleading error responses in metal-api

[Gerrit91]

We only set return code in case an audit error occurs and interrupt the request chain. If this happens the metal-api does not send back an error response JSON object, which was defined in the Swagger spec. Therefore errors from metal-go will be outputted like this:

Received unexpected error:
[DELETE /v1/machine/{id}/free][500] freeMachine default "{\"statuscode\":0,\"message\":\"\"}"

[Gerrit91]

A way to improve this would be to allow passing an error callback function, which is called in case an error occurs in the filter like func(err error, response resp *restful.Response) such that the caller can send the appropriate error response.