Add serialVersionUID to exceptions

Those classes are technically serializable, so they need a UID.
Fix infinite recursion in flatfile importer

The importer called a method that tried to import chests, which caused
a stack overflow error. BetterEnderFileHandler has been cleaned up so
that these kind of errors are less likely in the future. All existing
methods remain, but are deprecated.

Also improved error handling in importers.
Deprecate all uuid conversion classes and methods

All those classes and methods will be removed in the future.