Change generator to warn about invalid argument names

I had the following in my descriptor.json file:

          "args": {
            "result": {
              "isReturn": true
            },
            "opts:": {
              "isOptional": true
            }

I ran npm run generateJson and npm run generateNativeCode many times but the opts argument wouldn't become optional in the generated C++ file. I finally realized I made a typo. I named the argument opts: instead of opts! :(

We should change the generator so that it will generate warnings if a specified argument's name doesn't match what's declared in libgit2.