Add the case for when a recaptcha challenge fails but the request succeeds.

Author: Samuel Seay

README.md

@@ -88,7 +88,7 @@ Recaptcha provides the `verify/2` method. Below is an example using a Phoenix co
 
 `{:ok, %Recaptcha.Response{challenge_ts: timestamp, hostname: host}}` -> The captcha is valid, see the [documentation](https://developers.google.com/recaptcha/docs/verify#api-response) for more details.
 
-`{:error, errors}` -> `errors` contains atomised versions of the errors returned by the API, See the [error documentation](https://developers.google.com/recaptcha/docs/verify#error-code-reference) for more details. Errors caused by timeouts in HTTPoison or Poison encoding are also returned as atoms.
+`{:error, errors}` -> `errors` contains atomised versions of the errors returned by the API, See the [error documentation](https://developers.google.com/recaptcha/docs/verify#error-code-reference) for more details. Errors caused by timeouts in HTTPoison or Poison encoding are also returned as atoms. If the recaptcha request succeeds but the challenge is failed, a ``:challenge_failed` error is returned.
 
 `verify` method also accepts a keyword list as the third parameter with the following options:
 

lib/recaptcha.ex

@@ -41,6 +41,8 @@ defmodule Recaptcha do
         {:error, Enum.map(errors, fn(error) -> atomise_api_error(error) end)}
       {:ok, %{"success" => true, "challenge_ts" => timestamp, "hostname" => host}} ->
         {:ok, %Recaptcha.Response{challenge_ts: timestamp, hostname: host}}
+      {:ok, %{"success" => false, "challenge_ts" => timestamp, "hostname" => host}} ->
+        {:error, [:challenge_failed]}
     end
   end