vgsplit: Improve message when LV still active.

vgsplit: Improve message when LV still active.

Mention parent LV as well as the LV triggering the warning.

Still leaves some confusing cases but its not worth fixing them
at the moment.
(Thin pool inactive but a thin volume active => deactivate thin vol.
Inactive mirror/raid with pvmove in progress => complete pvmove and
active&deactivate mirror/raid.
If new VG already exists it requires some LVs to be inactive
unnecessarily.)