Missing exercises and nutshell pages

[tshiya731]

1. Missing exercises:

• Chapter Errors
• Chapter Sympy

2. Missing nutshells:

• Chapter Errors
• Chapter Sympy (there is a notebook named sympy_stripped.ipynb, i dont know if it's the nutshell?)
  @rlanzafame

[rlanzafame]

the _stripped refers to a specially formatted file for download by students that has the jupyter book syntax stripped out to make it more readable. this is good enough for now!