Fix small bug in external plugins

* Fix the variable to display in log when an external plugin fails to load.
* Add functional tests to explore external plugins in two
bad scenarios: 1) plugin without configure() method implemented,
2) plugin with syntax error.
What if we also had a plugin content file that has is syntactically correct, but doesn't have a class that inherits from `Plugin`?

Would it be worth to have a unittest that checks if that class is indeed not loaded?