Log RuntimeExceptions during rendering so that they are at least visible somewhere...

Log RuntimeExceptions during rendering so that they are at least visible somewhere if someone doesn't set an ErrorListener on the XSLT processor (this could lead to exceptions being swallowed). This is a temporary measure until we have time to improve the exception handling in FOP.
The original problem that led to this change was: "The number of this PDFNumber must not be empty" while producing a PDF/A with customized filter entries in the PDF configuration. The exception was a follow-up exception after a PDFConformanceException that was swallowed by a default ErrorListener.

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@600467 13f79535-47bb-0310-9956-ffa450edef68