Pre-validates paths for unregistered namespaces

- Adds FedoraInvalidNamespaceException type
- New FedoraInvalidNamespaceExceptionMapper
- Add new validatePath method to AbstractService
- Fixes Unit Test that require a mock NamespaceRegistry

This resolves https://www.pivotaltracker.com/story/show/61500426
I will change the class this logger is tracking for you.