Move automation engine code under "Engine" namespace

This is to separate the engine itself from "integrations" that will be
built on top of the engine.

[MAILPOET-4136]

mailpoet/lib/Automation/Engine/API/ErrorResponse.php

@@ -0,0 +1,18 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\Engine\API;
+
+class ErrorResponse extends Response {
+  public function __construct(
+    int $status,
+    string $message,
+    string $code
+  ) {
+    parent::__construct(null, $status);
+    $this->set_data([
+      'code' => $code,
+      'message' => $message,
+      'data' => ['status' => $status],
+    ]);
+  }
+}