Don't assume history request succeeded

errors in startup can cause the history request to be aborted.  There could be any of
a number of reasons the history request should fail, and the frontend should *never*
assume that the status is 'ok'.

IPython/frontend/qt/console/ipython_widget.py

@@ -174,7 +174,11 b' class IPythonWidget(FrontendWidget):'
         """ Implemented to handle history tail replies, which are only supported
             by the IPython kernel.
         """
-        history_items = msg['content']['history']
+        content = msg['content']
+        if 'history' not in content:
+            self.log.error("History request failed: %r"%content)
+            return
+        history_items = content['history']
         items = [ line.rstrip() for _, _, line in history_items ]
         self._set_history(items)