Stop emitting error and non-error response simultaneusly in envoy authz Check

[krdln]

Eg.

aperture/pkg/policies/flowcontrol/api/envoy/authz.go

Lines 168 to 173 in a642494

	checkResponse := &flowcontrolv1.CheckResponse{
	Error: flowcontrolv1.CheckResponse_ERROR_CONVERT_TO_MAP_STRUCT,
	Services: svcs,
	}
	resp := createExtAuthzResponse(checkResponse)
	return resp, fmt.Errorf("converting raw input into rego input failed: %v", err)

Grpc server ignores the response if the error is not nil, so this response is not used in any way. We want to track the exact error reason though, preferably through metrics / logs (see also #882).

Would also be nice if returned errors wouldn't be just strings, but something that works with https://pkg.go.dev/google.golang.org/grpc/status#FromError, so that grpc server can send a meaningful response code.