Throw if XLSX Writer configured after being opened

src/Spout/Writer/Exception/WriterAlreadyOpenedException.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace Box\Spout\Writer\Exception;
+
+/**
+ * Class WriterAlreadyOpenedException
+ *
+ * @package Box\Spout\Writer\Exception
+ */
+class WriterAlreadyOpenedException extends WriterException
+{
+}