Add error messages to action validation, use step validation args

[MAILPOET-4659]

mailpoet/lib/Automation/Engine/Integration/ValidationException.php

@@ -0,0 +1,8 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\Engine\Integration;
+
+use MailPoet\Automation\Engine\Exceptions\UnexpectedValueException;
+
+class ValidationException extends UnexpectedValueException {
+}