fix loading message outputs when loaded as plugins

ClassNameStringValidator assumes that the default classloader can see all classes, which is not the case for jar plugins, in which it fails.
Since we check for loadable classes later on, and skip those that aren't there (anymore) we don't need to validate the specific class name at this point, in fact, this would need access to the set of plugins which isn't available in this piece of code and is tricky to get in, because none of the code is using dependency injection

fixes #730