qtconsole --pure, avoid 'user_expression'

	'user_expression' does not exist with --pure, be more carefull..

IPython/frontend/qt/console/frontend_widget.py

@@ -370,7 +370,9 b' class FrontendWidget(HistoryConsoleWidget, BaseFrontendMixin):'
 
         """
 
-        user_exp = msg['content']['user_expressions']
+        user_exp = msg['content'].get('user_expressions')
+        if not user_exp:
+            return
         for expression in user_exp:
             if expression in self._callback_dict:
                 self._callback_dict.pop(expression)(user_exp[expression])