Objectpos over limit: Avoid crash caused by sector over limit

Objectpos over limit: Avoid crash caused by sector over limit

Reduce the object limit by mapblock size, to avoid objects being
added just inside the map generation limit but in a block and sector
that extend beyond the map generation limit.

Change notification of 'objectpos over limit' from red in-chat ERROR
to in-terminal only WARNING, since this will happen often using mob
mods near the world's edge.