Create Err variable so a user can pass a traceback to the LLM

[JasonWeill]

Problem

Jupyter notebook authors would like to pass an error generated by their code to a large language model (LLM) so that they can understand the error better.

1. User runs code
2. Code throws an error
3. User wants to ask the LLM what the error means, without copying and pasting the error into a new prompt

Proposed Solution

Create a special Err list variables, with the same indexes as the In and Out variables provided by IPython. When a user runs code in cell In[n] that raises an uncaught exception, information about that exception is captured, as a string, in Err[n], and still displayed in the output area.