Add error_user_title and error_user_msg fields in APIError

In some case, Facebook API includes `error_user_title` and `error_user_msg` with more descriptive message that can be used to know which parameter is invalid and why. 
Example:
```json
{
  "error": {
    "message": "Invalid parameter",
    "type": "FacebookApiException",
    "code": 100,
    "error_subcode": 1349118,
    "is_transient": false,
    "error_user_title": "Disallowed Post Link",
    "error_user_msg": "One or more of the given URLs is not allowed by the Stream post URL security app setting. It must match the Website URL or Canvas URL, or the domain must be a subdomain of one of the App's domains. Read https://developers.facebook.com/docs/facebook-login/security/ to learn more about app settings related to security."
  }
}
```

Including this message is way more usefull than just having the `error.message` value.