Make NAT port forward data entry error message match GUI text

The GUI has "Redirect target IP" and "Redirect target port". But the error message when validating "Redirect target port" refers to it as "local port". I was just confused when I forgot to type in the "Redirect target port", the error told me I had to enter a valid local port. I looked down the GUI page for "local port" and it is not there.
Merge pull request #741 from phil-davis/master

Make NAT port forward data entry error message match GUI text