remove docstrings from TestLinkErrorHandlerMethods

apparently they replace the method name during nosetest prints
Don't rely on _categorize_errors in test_process_errors
make indentation uniform
make link to append to error message a class member
make __repr__ use class members
add __str__ which delegates to __repr__
ExternalLinkage.summary_mesage: fall back to super's
ExternalLinkage: implement own prefix, link, description