feat(instant): invoke missing handler within `$translate.instant(id)`

Unless a translation was found, the `$missingTranslationHandlerFactory` will now be invoked (was already part of the async part)
Yes, some gap / missing code there. Damn... had it on my list in March, but forgot about it somehow :-(. 
Should be extended to contain the block with the return value: 
see https://github.com/angular-translate/angular-translate/blob/master/src/service/translate.js#L1057-L1061

As there are some more occurrences, we should make a function out of it I would say.