added API/Error class to hold error keys as constants

- re-added Setting::getAll() to API/Setting::set() in response
- updated settings/setup tests

lib/API/Endpoints/Settings.php

@@ -1,6 +1,7 @@
 <?php
 namespace MailPoet\API\Endpoints;
 use \MailPoet\API\Endpoint as APIEndpoint;
+use \MailPoet\API\Error as APIError;
 use \MailPoet\Models\Setting;
 
 if(!defined('ABSPATH')) exit;
@@ -10,18 +11,20 @@ class Settings extends APIEndpoint {
   }
 
   function get() {
-    $settings = Setting::getAll();
-    return $this->successResponse($settings);
+    return $this->successResponse(Setting::getAll());
   }
 
   function set($settings = array()) {
     if(empty($settings)) {
-      return $this->badRequest();
+      return $this->badRequest(array(
+        APIError::BAD_REQUEST =>
+          __("You have not specified any settings to be saved.")
+      ));
     } else {
       foreach($settings as $name => $value) {
         Setting::setValue($name, $value);
       }
-      return $this->successResponse();
+      return $this->successResponse(Setting::getAll());
     }
   }
 }

lib/API/Endpoints/Setup.php

@@ -17,7 +17,7 @@ class Setup extends APIEndpoint {
       return $this->successResponse();
     } catch(\Exception $e) {
       return $this->errorResponse(array(
-        $e->getMessage()
+        $e->getCode() => $e->getMessage()
       ));
     }
   }