sessionservice: Introduce a well know "not exists" to sessionservice

Most of the methods in the Service interface can fail if the session doesn't already exist:

```
type Service interface {
Get(context.Context, *GetRequest) (StoredSession, error)
Delete(context.Context, *DeleteRequest) error
AppendEvent(context.Context, StoredSession, *session.Event) error
}
```

Introduce a well known error from the package, e.g.`ErrSessionNotFound`, to let the clients handle this case easily.