Wrap Plugin.getDefaultWorldGenerator() in try-catch

### Problem
If a plugin throws an exception in getDefaultWorldGenerator, it will propagate higher up in the server, with possibly disastrous effects.

### What this PR does
This PR contains the changes from #1082, plus we use the full plugin name in the logging and catch the exception.

## Links
CraftBukkit#1082
Bukkit/Bukkit#568
https://bukkit.atlassian.net/browse/BUKKIT-2565
https://bukkit.atlassian.net/browse/BUKKIT-4116