make TestLinkError inherit from LinkError
make RecipeCorrectButBuildScriptBroken inherit from BrokenLinkage

instead of ExternalLinkage, since fatal should be false
use multiple inheritance for LinkError tests

so LinkError memebers, like full_message, can be referenced
TestLinkErrorHandler: properly name (currently empty) tests
LinkErrorHandler: move all error_message creation to _process_errors
LinkErrorHandler: make assert_disjoint a module method

instead of defining it inline