implement TwingleShop integration

Civi/Twingle/Shop/Exceptions/LineItemException.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace Civi\Twingle\Shop\Exceptions;
+
+use Civi\Twingle\Exceptions\BaseException as BaseException;
+
+/**
+ * A simple custom exception that indicates a problem within the Line Items
+ */
+class LineItemException extends BaseException {
+
+  public const ERROR_CODE_CONTRIBUTION_NOT_FOUND = "contribution_not_found";
+
+}