* refactor(errorsx): start hiding private details and moving around stuff
Part of https://github.com/ooni/probe/issues/1505
* fix: remove now-addressed todo comments
I needed to add some tests as integration tests due to circular
imports, but this is ~fine because we quite likely want many
integration tests in the errorsx package anyway.
Part of https://github.com/ooni/probe/issues/1505.