Clarify error message

Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/models/subproject.py

@@ -1215,7 +1215,7 @@ class SubProject(models.Model, PercentMixin, URLMixin, PathMixin):
         elif self.new_lang != 'add' and self.new_base:
             raise ValidationError(_(
                 'Base file for new translations is not used because of '
-                'project settings.'
+                'project settings (adding new translations is disabled).'
             ))
 
     def clean_template(self):