Fix upgrade stdout handling if file doesn't exist yet

plus some other tweaks..

tailbone/views/master.py

@@ -895,6 +895,10 @@ class MasterView(View):
                 progress.session['success_url'] = success_url
                 progress.session.save()
 
+    def execute_error_message(self, error):
+        return "Execution of {} failed: {}: {}".format(self.get_model_title(),
+                                                       type(error).__name__, error)
+
     def get_execute_success_url(self, obj, **kwargs):
         return self.get_action_url('view', obj, **kwargs)