improve error handling during invoice generation (#2932)

* prevent that items will be marked as exported if invoice can not be generated
* check for existing invoice number to prevent that invalid invoices will be generated
* allow to add a unique id to invoice preview files on batch export
* hide delete invoice action with deactivated permission
* try to automatically fix duplicate invoice id

src/Invoice/DuplicateInvoiceNumberException.php

@@ -0,0 +1,18 @@
+<?php
+
+/*
+ * This file is part of the Kimai time-tracking app.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace App\Invoice;
+
+final class DuplicateInvoiceNumberException extends \Exception
+{
+    public function __construct(string $invoiceNumber)
+    {
+        parent::__construct('Invoice number "' . $invoiceNumber . '" already existing');
+    }
+}