Cleaner response for captcha-related createaccount API failure

Instead of a generic API error, you get back a non-Success-resulted
creataccount response, with result='NeedCaptcha'. There's also a warning
included with the message key, and of course the captcha key:

array (
  'createaccount' =>
  array (
    'result' => 'NeedCaptcha',
    'warnings' =>
    array (
      0 =>
      array (
        'type' => 'warning',
        'message' => 'captcha-createaccount-fail',
        'params' =>
        array (
        ),
      ),
    ),
    'captcha' =>
    array (
      'type' => 'simple',
      'mime' => 'text/plain',
      'id' => '91510936',
      'question' => '76−3',
    ),
  ),
)

Change-Id: Id1c9e387c592e6d51a5bd58d99ce3d644dfa300b
Merge "Cleaner response for captcha-related createaccount API failure"