Missing error.source for #6

**version:**
1.0.0-beta.6

**input:**

```yaml
abc: 123
def
```

**output:**

```
YAMLSemanticError {
  name: "YAMLSemanticError",
  message: "Implicit map keys need to be followed by map values", 
  source: undefined
}
```

**expected:**

`error.source` exists.