Improvements for page events

Improve the implementation of the Page events.

From PCentgraf's comments on HSTTWO-4177:

1. There's some inconsistency in the API, sometimes providing JCR nodes, sometimes HST model objects, and in one case a JCR path String. It would be better to be more consistent.

2. We should make sure that the Channel Manager responds in a reasonable way if an event is canceled on the backend. At minimum, the CM should show that the change didn't really happen. Ideally, there should be a translated UI message that indicates that the change was rejected by the backend for some reason – i.e. not just the RuntimeException message. And there should be some UI tests in the HAP to verify the behavior.