Fix bug 1437989: Identify broken translations with error message (#859)
Fix bug 1437989: Identify broken translations with error message (#859)

This fixes these kinds of bugs:

If the string is untranslated and we only translate one of the two
variants in the rich editor, the unsaved changes warning is not
displayed.
mozilla/pontoon,jotes/pontoon,mozilla/pontoon,mozilla/pontoon,mathjazz/pontoon,jotes/pontoon,jotes/pontoon,mathjazz/pontoon,mathjazz/pontoon,jotes/pontoon,mozilla/pontoon,mozilla/pontoon,mathjazz/pontoon,mathjazz/pontoon