feat(missingFallbackDefaultText): enables a feature to return a default ...

...text for displaying in case of missing translations including fallback stack

In case you have defined a missingTranslationHandlerFactory, you now have the possibility to define a return value which is set instead of the translationId.
This gives a lot of flexibility for certain use cases.
Example factory:
```
        $provide.factory('customTranslationHandler', function () {
          return function (translationID, uses, fallbacks) {
            if (fallbacks === true) {
              return 'NO KEY FOUND';
            }
          };
        });
```