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
Needs is a tricky word. I used nodeExists because it was the first place to fail, so was the place to catch the bad paths before they turned in to RepositoryExceptions. I didn't encounter any other spots, but I'm not sure if that means i solved them all or just didn't locate them. I'll try out a couple more operations tomorrow to ensure they aren't skipping this check .

Also, I submitted this PR without completing the task of having the HTML ui allow for a place to register the namespace. My apologises I dropped the ball on that.