Merge branch 'exception-handler' into 'master'

Add new Exception handler

While working on #732 I again needed to use a `UiResultExceptionHandler`  when I actually don't need to return a result. We have some other places in the code like this. So I introduced a `UiExceptionHandler` without the result part and used it where appropriate. While I was touching some classes, I also added the new annotations.

This MR includes another small commit that notifies only on local group messages. Joining a group is a message as well and without this change, you are notified about a new message when you yourself joined your newly created group.

See merge request !403