Collect the test whether a plane fits on a carrier in one place:

Collect the test whether a plane fits on a carrier in one place:
(fit_plane_on_ship, fit_plane_on_land): New.
(pln_oneway_to_carrier_ok, put_plane_on_ship, count_planes)
(put_plane_on_land, count_land_planes): Use them.  No functional
change.
(fit_plane_off_ship, fit_plane_off_land): New.
(take_plane_off_ship, take_plane_off_land): Use them.  This oopses
when the carriers plane counter underflows.

(take_plane_off_ship): Document that carrier's plane counters may be
screwed up.

(take_plane_off_ship, take_plane_off_land): Oops when the plane isn't
on the carrier.

(take_plane_off_ship, take_plane_off_land): Don't fail when the plane
to be taken off can't go on this type of carrier, just take it off.
No error condition left, so return void.  Callers couldn't do anything
useful with the status anyway, and most didn't bother.  Change those
that did.