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
Use @Test(expected=MyException.class).

See [here](http://junit.org/apidocs/org/junit/Test.html).