Error message from fastjson::dom::parse_string is always empty.

Errors might be reported by the parsing code. and spewed to the console, but the actual string that should hold the error message is always empty.

```
OMG an error [4003] : Unexpected character while parsing array
It seems to have happened here...
}]
```

   ^
   Unable to parse json : 

When trying to parse this json

```
[{},{}}]
```
