Add LinkageError amd NullPointerException exception.

It fixes SVG export on sweethome3d <http://www.sweethome3d.com>
See
  http://sourceforge.net/p/sweethome3d/bugs/338/
  http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=657700

Even applying https://github.com/freehep/freehep-vectorgraphics/pull/18 would be good.

Thanks for considering