externalize Success/ErrorResponse classes into their own files

lib/API/APIErrorResponse.php

@@ -0,0 +1,34 @@
+<?php
+namespace MailPoet\API;
+
+if(!defined('ABSPATH')) exit;
+
+class APIErrorResponse extends APIResponse {
+  public $errors;
+
+  function __construct($errors = array(), $meta = array(), $status = self::STATUS_NOT_FOUND) {
+    parent::__construct($status, $meta);
+    $this->errors = $this->formatErrors($errors);
+  }
+
+  function getData() {
+    if(empty($this->errors)) {
+      return false;
+    } else {
+      return array(
+        'errors' => $this->errors
+      );
+    }
+  }
+
+  function formatErrors($errors = array()) {
+    $formatted_errors = array();
+    foreach($errors as $error => $message) {
+      $formatted_errors[] = array(
+        'error' => $error,
+        'message' => $message
+      );
+    }
+    return $formatted_errors;
+  }
+}