Improved TransformationFacade error handling
* ErrorHandler class created in 5859cb6 now caches all errors instead of pushing them to std::cerr
** cached errors are retrieved by TransformationFacade's "generate" member method
* test frontend pushes all errors to std::cerr
* FunctionTransform returns errors to the calling template as XML
** FunctionTransform test case demonstrates how one may test for successful transformation
* "generate" member method returns std::string vector wrapped in a std::unique_ptr
** this is used as a kind of optional pointer, as the std::unique_ptr instance only wraps a vector if errors where actually generated