New Translator function: getNotCanTranslateReason()

while working on supertext module I wanted to show the user a better explanation why his Job cannot be translated by the selected translator.
There is such a reason function for isAvailable: getNotAvailableReason() but not for the canTranslate function.

the attached patch adds this to tmgmt core.

The whole thing also depends on #1420364: A simple response object for methods which need to return a true/false *and* a message which then makes the reason functions obsolete. But until then I think we can add this.