instead of crashing on functions, print something a little more useful

Right now, if you call `json(some_function)` you might get a stack overflow or you might get a nice message that says "ERROR: access to undefined reference". I've definitely even crashed Julia this way >.<

I don't know of any good way to really convert functions into json, but I think we should at least not error (or error in a more recoverable way). I figure this gives some useful information.