Look at on_altair_error() to see we can print out the original error message; then temporarily set dist: trusty on Travis to make sure we get a descriptive error.

The current readme.md has explanations about how to write python code that does the things we want. It needs a new section, directed at people who only want to use the code and never want to look inside the code. You might have more than one .md file if you want, and link them together, that's fine, but here's the kinds of things we want it to cover:

1. Overall flow: simulation -> JSON stats -