Resolved gh-444 by adding the new phpspreadsheet library, and removing the old PHPExcel library.

libraries/phpspreadsheet/vendor/markbaker/complex/classes/src/Exception.php

@@ -0,0 +1,13 @@
+<?php
+
+/**
+ * Exception.
+ *
+ * @copyright  Copyright (c) 2013-2018 Mark Baker (https://github.com/MarkBaker/PHPComplex)
+ * @license    https://opensource.org/licenses/MIT    MIT
+ */
+namespace Complex;
+
+class Exception extends \Exception
+{
+}