Error code now correctly set in FacebookAPIException

Currently the error codes from Graph API calls do not get set in the FacebookApiException object because there is no 'error_code' in the error response.

I made it so that when it is in the "OAuth 2.0 Draft 00 style" block, it assigns $code the value of ['error']['code']. I do this because when ['error']['message'] exists, ['error']['code'] exists as well.