build: Report missing stuff more nicely for ship, plane, land units

build: Report missing stuff more nicely for ship, plane, land units

If both materials and avail are missing, report missing avail instead
of materials, to avoid tempting the player to move in materials only
to discover avail is lacking, too.

Report what materials are missing instead of just "Not enough
materials".  Does not yet include military for planes, but that's
next.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>