Ensure that result is defined in run_cell()

This can obscure underlying errors

IPython/core/interactiveshell.py

@@ -2659,6 +2659,7 b' class InteractiveShell(SingletonConfigurable):'
         -------
         result : :class:`ExecutionResult`
         """
+        result = None
         try:
             result = self._run_cell(
                 raw_cell, store_history, silent, shell_futures)