Implement error url for File controller

tracim/tracim/controllers/content.py

         return tg.url('/workspaces/{}/folders/{}')
 
     @property
+    def _err_url(self):
+        return tg.url('/dashboard/workspaces/{}/folders/{}/file/{}')
+
+    @property
     def _item_type(self):
         return ContentType.File