DERBY-6493: Improve reporting of exceptions wrapped in InvocationTargetException

Unwrap InvocationTargetException and PrivilegedActionException before
reporting them so that it is easier to see what the real error is.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1574425 13f79535-47bb-0310-9956-ffa450edef68