LengthNormUpdater returning error code if no fields in index have norms is inconvenient.

As I started using the LengthNormUpdater in AIT, I ran into situations where returning an error code of 0 if no fields in the index had norms was a hassle. In the scripts using the LNU, distinguishing real errors from this condition is important, and using a return code of 0 for both prevented that.