window: Emit an error and return when trying to activate an unmanaged
As per followup of this discussion: !562 (comment 502846)
If something (i.e. gnome-shell or an extension) tries to activate an unmanaged window, we should warn about this and avoid to perform further actions as this could lead to a crash of mutter, since the window has not valid flags (like workspace) set anymore at this stage.
Fixes #580 (closed)