fairland: Fail when island can't be placed, for fairness

fairland: Fail when island can't be placed, for fairness

The previous commit made island distribution more fair by placing
islands close to a continent in turn.  This is still unfair when
fairland can't place all the islands.

Make grow_islands() fail when it can't place all islands, and main()
start over then, just like it does when grow_continents() fails.

Deities can no longer fill the world with islands by asking for a
impossibly high number of islands.  Tolerable loss, I think.

fairland-test needs a tweak to avoid loss of test coverage.

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