Missing Structure Fix

Fixes MC-194811 - A bug that stops chunks from saving if they have missing structures and improves the logging when a missing structure is found. https://bugs.mojang.com/browse/MC-194811

Missing structures are caused by either removing mods that add structures or mods removing structures themselves.

Any chunks with missing structures will be unable to save to disk effectively leaving it in a read only state, this mod allows those chunks to save normally.

Removing this mod before visiting all chunks with missing structures will lead to the chunk being unable to save again.

Bugfix ported from Fabric API, adapted to Forge