Improve properties of ValidationError

Improve properties of ValidationError:
- change `statusCode` from 400 to 422
- nest `context` and `codes` inside `details`
- add `details.messages`
- reword the main error message

Remove the call to Error's constructor from ValidationError constructor, because it's a no-op - Error's constructor creates a new instance when called via `.call()`.

/to: @raymondfeng or @ritch: please review
/cc: @Schoonology 

Close strongloop/loopback#21

**A sample response**

```json
{
  "error": {
    "name": "ValidationError",
    "status": 422,
    "message": "The Model instance is not valid. See `details` property of the error object for more info.",
    "statusCode": 422,
    "details": {
      "context": "todo",
      "codes": {
        "owner": [
          "presence"
        ]
      },
      "messages": {
        "owner": [
          "can't be blank"
        ]
      }
    },
    "stack": "ValidationError: The Model instance is not valid. See `details` property of the error object for more info..\n    at [etc.]"
  }
}
```