Better traceback output for runtime errors

[acbart]

I'm thinking we should make the following changes:

1. Flip the order so that the callstack starts at the top instead of the bottom.
2. Have markdown format for each line:

• Bullet points for each row of the traceback
• Highlight the python code lines
• Make the actual error itself more separated out

3. Make sure that the explanatory text appears
4. Put the "When I tried running X" at the top.

[acbart]

This happens in the newest version. I didn't end up flipping the callstack, which I suppose is worth considering. It's also not producing HTML and Text, but I think that requires more thought.