Add static factory class for easier exception creation

[MAILPOET-4135]

mailpoet/lib/Automation/Exceptions.php

@@ -0,0 +1,13 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation;
+
+use MailPoet\Automation\Exceptions\InvalidStateException;
+
+class Exceptions {
+  public function __construct() {
+    throw new InvalidStateException(
+      "This is a static factory class. Use it via 'Exception::someError()' factories."
+    );
+  }
+}