daemon: only fatal if err != nil
daemon: only fatal if err != nil

This commit avoids cilium-agent from fatal itself on an expected exit
signal such as:

```
cilium-agent[11272]: level=info msg="Stopped serving cilium at unix:///var/run/cilium/cilium.sock" subsys=daemon
cilium-agent[11272]: level=fatal msg="Error returned from non-returning Serve() call" error="<nil>" subsys=daemon
```

Fixes: 894365fc6b0e ("daemon: Handle listen/serve errors with channels")
Signed-off-by: André Martins <bc9800b9d52a24cce72a73dd528afed53f10e5fc@cilium.io>

cilium-team/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,cilium-team/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,tgraf/cilium