Fixed the SPARQL node type deletion bug to return a clear error message from the backend.

https://www.pivotaltracker.com/s/projects/684825/stories/72805066

I think I've fixed it to return a clear error message for deleting a not assigned registered node type now. Attached is the screen shot with the error message:
![screen shot 2014-07-11 at 9 46 25 am](https://cloud.githubusercontent.com/assets/2430784/3555873/5b7c3f56-091c-11e4-9336-d7061e7a55bf.png)
http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Strings.html#isNullOrEmpty(java.lang.String)

is better.