notebook: limit stack trace to user's code

[switham]

If the user gets an error or exception in their Python code, they get a stack trace, which is great, but it extends up beyond their code into the pyinthephone code, which is both distracting and a little bit of a security issue. Trim away the extra.

The relatively new Python standard "code" module may provide some help in this, although, skimming the documentation today, I didn't see anything obviously helpful. Maybe a helpful trick can be borrowed from the source of "code". (What a dumb name.)

(Trimming the stack and using "code" were both Ned Batchelder's suggestions.)