Pre-validates paths for invalid namespaces

- Adds FedoraInvalidNamespaceException type
- New FedoraInvalidNamespaceExceptionMapper
- Modifies Service to add new validatePath method
- Call validatePath from AbstractService.nodeExists function
- Fixes Unit Test that require a mock NamespaceRegistry

This resolves https://www.pivotaltracker.com/story/show/61500426
Here is an example of the machinery that Junit provides for expecting exceptions:
https://github.com/fcrepo4/fcrepo4/blob/master/fcrepo-kernel-impl/src/test/java/org/fcrepo/kernel/impl/services/TransactionServiceImplTest.java#L120