[B+C] Make CraftBukkit send the world generator warning, not Bukkit

Only plugins are going to be calling this method. Thus, only plugins should be notified of the result (null). No need to tell the user that too.
See MultiVerse for an example of where this is used in practise.

This PR does change the message, though.

https://bukkit.atlassian.net/browse/BUKKIT-2565
https://github.com/Bukkit/Bukkit/issues/568