[stable10] Notify user while adding an existing group again
[stable10] Notify user while adding an existing group again

When user tries to add group from UI which already
exists, then a notification in the UI will help user
understand the cause for not adding.

Signed-off-by: Sujith H <f003c101b6db321bb70f3b324d211f3cd092837a@owncloud.com>

owncloud/core,owncloud/core,owncloud/core,phil-davis/core,owncloud/core,owncloud/core