Add cell tag to suppress our special messages

[davidwagner]

We should provide a way to mark a cell as exempt from our custom error messages. Suggested mechanism: if the cell has the tag "no-custom-error", then we don't display any of our custom error message, and just display the standard Python exception text that they'd see in teh absence of our library.

(You can add a tag to the Jupyter notebook by using Edit >> Cell Toolbar >> Tags.)

This will be useful for cells where we want to show erroneous code and show the standard Python exception message, without our custom additions.