Rewrote error handling based on exceptions
* ErrorHandler is replaced by ErrorCapacitor
** temporarily registers itself as both the ProblemListener and ErrorHandler of a given XalanTransformer instance
** deregisters itself on destruction
** collects all problems and errors during its lifetime inside a internal std::vector<std::string> instance
** if this instance is not empty it is thrown contained within a ErrorCapacitor::exception by the member method ErrorCapacitor::discharge
* this enables using the same code for handling transformation compilation and generation errors and problems
* updated test frontend accordingly