Avoid log flooding with messages about missing missing registry/services

Prevents this to appear in log every 2s:

```
26.314664 13972 etcd.go:118] Failed to get the key registry/services: 100: Key not found (/registry/services) [23]
26.314689 13972 etcd.go:78] Failed to get any services: 100: Key not found (/registry/services) [23]
```
So if the endpoints were deleted from etcd, then there are truly no endpoints.  You don't need to log in that case.  If any other error occurs, we need to go to the next service (continue).  So seems like it should be:

```
if err != nil  {
  if !isEtcdNotFound(err) {
    log
    continue
  }
  endpoints = []api.Endpoints{}
}
```