REP-819 Keystone V3 Filter

The mess that is Keystone v3! Comment on _everything_ so we can get this fixed up. =)
you can very easily create an exception with a message doing something like this:

`case class SomeException(message:String, cause:Throwable = null) extends Exception(message, throwable)`

Unless there's special logic in the exception, this is all you need to do.

At which point you can `throw SomeException("derp")`