return a form with validation errors in the same way as Exceptions...

I started this PR after asking (https://github.com/FriendsOfSymfony/FOSRestBundle/issues/541) what the best way was to handle api responses with non-valid forms.

Personally I don't like how it returned out of the box:

```javascript
{
  "children": {
    "username": {
      "errors": [
        "This value should not be blank."
      ]
    }
  }
}
```
I wanted something that could be incorporated into the error messages you get when calling

```php
throw new HttpException(400, "New comment is not valid.");
```
```javascript
{
  "status": "error",
  "status_code": 400,
  "status_text": "Bad Request",
  "current_content": "",
  "message": "New comment is not valid."
}
```
I changed a couple of files and was able to get following results if a non valid form is returned:

```javascript
{
  "status": "error",
  "status_code": 400,
  "status_text": "Bad Request",
  "current_content": "",
  "message": "Validation Failed";
  "errors": {
    "children": {
  	  "username": {
  	    "errors": [
  	  	"This value should not be blank."
  	    ]
  	  }
    }
  }
}
```

Let me know your thoughts about this.

Cheers