ZIP Import: Added upload handling

Split attachment service storage work out so it can be shared.

app/Exceptions/ZipValidationException.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace BookStack\Exceptions;
+
+class ZipValidationException extends \Exception
+{
+    public function __construct(
+        public array $errors
+    ) {
+        parent::__construct();
+    }
+}