reduce JVM heap memory consumed by group repository 404 not found responses

AbstractGroupRepository collects any throwables thrown from its member repositories when requesting content and stores them in a map (memberThrowables) which it then propagates upstream inside a GroupItemNotFoundException. The stored throwables are only used when handling 'describe' requests.

For complex setups with lots of nested groups and many concurrent requests these throwables can end up taking a significant amount of heap, only never to be used because the requests are almost exclusively non-describe requests.

By skipping collection of these throwables for non-describe requests we should reduce pressure on the heap for all users (those with complex setups should see the most benefit).