Prior to this change the server would crash when attempting to load a
chunk from a region with bad data.
After this change the server will defer back to vanilla behavior. At
this time, that means attempting to generate a chunk in its place
(and occasionally just not generating anything and leaving small
holes in the world).
Should Mojang choose to alter this behavior in the future, this change
will simply defer to whatever that new behavior is.
Zach Brown