Propogate error message from invalid child tab to the parent.

Change-Id: Ic3d571eebcb69ab424cef7a05f8595c40c0f4d05
Reviewed-on: https://git.eclipse.org/r/35838
Tested-by: Hudson CI
Reviewed-by: Roland Grunberg <rgrunber@redhat.com>
Tested-by: Roland Grunberg <rgrunber@redhat.com>