Refactor activity viewer endpoints #2811
Description
The discussion on #2785 (which is just the RawViewer endpoint) raises a few valid points about the structure of these endpoints:
- handle errors from the API calls
- validate the structure or content of event.data
- an inconsistency in how activity JSON is retrieved
Since these issues are present in other APIs, we didn't fix them in #2785 but should fix them for all API, in particular for ActivityViewer, which RawViewer is based on.