k8s: Don't error when CEP does not exist on endpoint exit
k8s: Don't error when CEP does not exist on endpoint exit

An endpoint may be deleted before the CEP was ever created. Skip logging any
errors if the error is NotFound and avoid returning an error as the controller
was successfully stopped either way.

This avoids errors like:
```
level=error msg="Unable to delete CEP" containerID=8ac6da9fb1 controller="sync-to-k8s-ciliumendpoint (963)" datapathPolicyRevision=43 desiredPolicyRevision=43 endpointID=963 error="ciliumendpoints.cilium.io \"couchdb-server\" not found" identity=15353 ipv4= ipv6= k8sPodName=staging-couchdb/couchdb-server subsys=endpointsynchronizer
level=warning msg="Error on Controller stop" consecutiveErrors=2 error="ciliumendpoints.cilium.io \"couchdb-server\" not found" name="sync-to-k8s-ciliumendpoint (963)" subsys=controller uuid=8845cc2c-5d17-11e9-916d-02b2f5c938d8
```

Signed-off-by: Thomas Graf <5f50a84c1fa3bcff146405017f36aec1a10a9e38@cilium.io>

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