Prioritize extracting validation messages when validation-domain is set

When creating a new Croogo release, in the app dir, I typicallyrun the following:

```
Console/cake i18n extract \
    --paths `pwd`/Vendor/croogo/croogo/ \
    --output /home/rachman/work/core/croogo-locale/ \
    --merge no \
    --validation-domain croogo \
    --extract-core no \
    --overwrite \
    --exclude Vendor/croogo/croogo/View,Vendor/croogo/croogo/Plugin,vendor 
```

It's suppose to extract the i18n messages to a separate git working directory for the locale repository. However, it is not extracting model validation messages.

This PR prioritize extracting validation messages to address the aforementioned use case.