Unpleasent behavior when enrolling to non existing policy

When agent we try to add fleet-server agent specifying `-fleet-server-policy` with an incorrect value (e.g typo) agent misbehaves.

It spins up fleet server, fleet server tries to read policy index with a specified policy which is empty, and retries and retries

behavior is not well defined:

```go
m.reporter.UpdateState(client.UnitStateStarting, fmt.Sprintf("Waiting on policy with Fleet Server integration: %s", m.policyID), nil) //nolint:errcheck // not clear what to do in failure cases
```

there may be a memory leak involved