sh: unbound variable error suppressed by status

Nash reports errors of unbounded variables, but when we get the status code of the command statement the error is being ignored.

```
λ> ls $vardonotexists
ERROR: <interactive>:1:3: Variable $vardonotexists not set on shell parent scope
λ> var _, status <= ls $vardonotexists
λ>  
```

That's unacceptable, because this is a programmer's error.