jurt: Throwable.getCause() instead of Throwable.getTargetException()

As of the JDK 1.4, Throwable.getCause() is the preferred method for getting thrown target exception.

Change-Id: I14f3f9ae52869d1149f92b32562e7fb3293109b5
Reviewed-on: https://gerrit.libreoffice.org/11027
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>