Fixed error handling of invalid XSL transformations
* xalan is segfaulting in XalanDOMString constructor when the static member "XalanLocator::getSystemId" is returning null-pointer
** this should seemingly be handled by passing a pointer to the static constant m_dummy but this sadly fails to prevent the segfault
** the patch provided by this commit fixes the problem for now but I cannot guarantee that this wont cause any side effects in a other context
* Adapted TransformationFacade to handle errors during transformation compilation
** errors generated during transformation compilation are returned through the generate member method
** throwing an exeception during TransformationFacade construction would require additional error handling in the calling code
* Added xalan::ProblemListener functionality to the ErrorHandler class
** XSLT problems passed to the ProblemListener are returned alongside to and treated as all other transformation errors