bad error message with list comprehension over object

Somewhere deep in my nested object, a list got accidentally replaced with an object.
Minimal working example:
```
jsonnet -e '[a for a in {}]'
```

The error I get with go-jsonnet is:
```
RUNTIME ERROR: Unexpected type object, expected array
builtin function
During evaluation
```
Without telling me what happened or even where. After a while I gave up and tried the c++ version, which gave me:

```
RUNTIME ERROR: In comprehension, can only iterate over array.
:1:1-16
```