Implemented error handling for XBlock instances

Failed XBlock loads previously logged a warning that went nowhere. They are now stored in a list of failed XBlock instances (with their exception). @pmitros @nedbat 
If someone is running this test while they have a bad XBlock instance (as an entry point), it should only succeed if there is a failed instance that is the one I'm looking for. The test not only ensures that there _can_ be failed plugins, but that `register_temp_plugin` is working correctly.