Swallow potential exceptions from showtraceback()

The nbgrader project is aware of a form of cheating where students
disrupt `InteractiveShell.showtraceback` in hopes of hiding exceptions
to avoid losing points. They have implemented a solution to prevent this
cheating from working on the client side, and have some tests to
demonstrate this technique:

https://github.com/jupyter/nbgrader/blob/main/nbgrader/tests/apps/files/submitted-cheat-attempt.ipynb
https://github.com/jupyter/nbgrader/blob/main/nbgrader/tests/apps/files/submitted-cheat-attempt-alternative.ipynb

In essence, these attacks import the interactive shell and erase the
traceback handler so that their failing tests won't report failures.

    import IPython.core.interactiveshell
    IPython.core.interactiveshell.InteractiveShell.showtraceback = None

The problem is that this causes an exception inside the kernel, leading
to a stalled execution. The kernel has stopped working, but the client
continues to wait for messages. So far, nbgrader's solution to this is
to require a timeout value so the client can eventually decide it is
done. This prevents allowing a value of `None` for `Execute.timeout`
because this would cause a test case to infinitely hang.

This commit addresses the problem by making `InteractiveShell._run_cell`
a little more protective around it's call to `showtraceback()`. There is
already a try/except block around running the cell. This commit adds a
finally clause so that the method will _always_ return an
`ExecutionResult`, even if a new exception is thrown within the except
clause. For the record, the exception thrown is:

    TypeError: 'NoneType' object is not callable

Accepting this change will allow nbgrader to update
`nbgrader.preprocessors.Execute` to support a type of
`Integer(allow_none=True)` as the parent `NotebookClient` intended.

Discussion about this is ongoing in jupyter/nbgrader#1690.

IPython/core/interactiveshell.py

@@ -3013,7 +3013,7 b' class InteractiveShell(SingletonConfigurable):'
             runner = _pseudo_sync_runner
 
         try:
-            return runner(coro)
+            result = runner(coro)
         except BaseException as e:
             info = ExecutionInfo(
                 raw_cell, store_history, silent, shell_futures, cell_id
@@ -3021,6 +3021,7 b' class InteractiveShell(SingletonConfigurable):'
             result = ExecutionResult(info)
             result.error_in_exec = e
             self.showtraceback(running_compiled_code=True)
+        finally:
             return result
 
     def should_run_async(