Use exit code 3 for invalid

This makes determining whether or not a result is valid more straightforward in a bash script

before
```sh
invalid=$(mapbox-upload-validate "$tmpfile" 2>&1 > /dev/null)
if [ "$invalid" != "" ]; then echo "File is invalid"; fi
```

after
```sh
mapbox-upload-validate "$tmpfile"
if [ "$?" == "3" ]; then echo "File is invalid"; fi
```