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
There is a way for users to add namespaces, see:
https://wiki.duraspace.org/display/FF/Feature+Tour+-+Namespaces

Probably giving a clue to that capability in the UI error message is adequate.