Explain error case when using function

[Robsteranium]

As per #11, cc @vsmart.

Does this mean e.g. when someone tries (total-bill "nan")? The error message will certainly be a bit cryptic the first time you encounter it. I wonder though if it would interrupt the flow to introduce it here.

btw I drafted some explanations of errors for the last event. In the end I put it into a blog post rather than the curriculum but I'd be happy to feed it in somehow.