Bug: Report back missing effect class

Author: einarf

demosys/effects/registry.py

@@ -82,7 +82,7 @@ def find_effect_class(self, class_name, package_name=None) -> Type[Effect]:
             if effect_cls:
                 return effect_cls
 
-        raise EffectError("No effect class '{}' found in any packages".format(effect_cls))
+        raise EffectError("No effect class '{}' found in any packages".format(class_name))
 
 
 class EffectPackage: