Improve webDAV 

Added event tracker on webDAV service.
btw note that there is a type erasure in this case...
so actually all three cases are the same in runtime:

```
case Failure(_, e: C3AccessException, _)          => throw new AccessDeniedException()
case Failure(_, e: GroupAccessDeniedException, _) => throw new AccessDeniedException()
case Failure(_, _, _)                             => null
```

I.e. first `case` will alway be evaluated in runtime...