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) {

          };
        });
docs (custom-error-handler): Add documentation for using default return value