Implemented ErrorMultiplexer as primary error handler
* ErrorMultiplexer is derived from both xercesc::ErrorHandler and xalan::ProblemListener
* registers itself as XalanTransformer's ErrorHandler and ProblemListener
* distributes captured errors and warnings to all registered ErrorMultiplexer::Receiver instances
** ErrorCapacitor implements the ErrorMultiplexer::Receiver interface and as such registers itself in a given ErrorMultiplexer instance
** ErrorMultiplexer reduces the different xalan and xercesc internal error classifications into either warnings or errors
* this was implemented to make it possible to easily differentiate between warnings and errors
** previously warnings were treated as errors
** ErrorCapacitor ignores warnings and only captures errors
** WarningCapacitor will be implemented to handle warnings during XSLT processing