[ENHANCEMENT] added methods to retrieve portals and urls from WebspaceManager

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| BC Breaks     | none
| Doc           | none
But if its only used in one place, I would write it explicitly -- it helps readability. Also if the Exception will be used in *more* than one place, then the situation has changed (or you are not being DRY) and the message should express that.

But the main thing is that you don't pass any message to the constructor here :)