Better error on mismatched binary operator

Consider this invalid snippet:

```jsonnet
{
abc: null
} + {
abc+: {
foo: "bar"
}
}
```

First, surprised me that it is invalid after all, because I assumed I could add `object` and `null` (which is represents "nothing", right?).

More importantly, the error message printed by `go-jsonnet` is pretty useless:
```
RUNTIME ERROR: Unexpected type null
During manifestation
```

While the one of `c++-jsonnet` is much better:
```
RUNTIME ERROR: binary operator + requires matching types, got null and object.
test.jsonnet:(3:5)-(7:2) object
During manifestation
```