`error` at the empty type

```
Cryptol> error "This is an error" : [0]
0x0
```

This is kind of strange. I have something that works like:

```
f a b = if a then error "oh no!" else b
```
And I want the error to occur when `a == True`, even when `b` has type `[0]'.

Though, would we need to be careful to keep `undefined` working as it currently does?