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
Also, could you please add an integration test for this. Possibly in:
https://github.com/fcrepo4/fcrepo4/blob/master/fcrepo-kernel-impl/src/test/java/org/fcrepo/integration/kernel/impl/services/NodeServiceImplIT.java