MessagesManager.DEFAULT_BASENAME currently refers to a file which is in fact in the admincentral module. This a completely soft dependency, so the worse that could happen is that those messages aren't found - however, for isolation, cleanliness and testability, this shouldn't be the case.
We probably don't even need a default messages at all - or maybe a very limited one with just a couple of messages about Magnolia (version, build and so on? - although these informations are also provided by LicenseFileExtractor)
Since I'm adding tests for MessagesManager, I'll add a fake resource file in the core/test folders to work around this for now.