Move File.exists logic in Config into the closure

This allows the dumpDir field to be easily overriden. Here is why:

The pull request to distributed-extraction-framework https://github.com/dbpedia/distributed-extraction-framework/pull/26 needs this. Take a look at https://github.com/dbpedia/distributed-extraction-framework/commit/70c90f19ba69e76b297431a513be493d7d943c6c#diff-ca3c67cce9806e13ddceb818917a5fdaR134 :

`override val dumpDir: File = null` makes dumpDir null, but overriding it does not stop `if (! dumpDir.exists) throw error("dir "+dumpDir+" does not exist")` (existing Config) to throw a NullPointerException.