Add error_user_title and error_user_msg fields in APIError

[isra17]

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:

{
  "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.

[isra17]

Just saw this PR after I created mine: #412 ...
Pretty much the same except mine include the new fields in the error message.

[nickhoffman]

This looks great. 👍

[arsduo]

Thanks! I'd merged in #412 already, so I manually added the code for the error message as you suggested.