Added option to provide custom slug when creating a short url

module/Core/src/Exception/NonUniqueSlugException.php

@@ -0,0 +1,14 @@
+<?php
+declare(strict_types=1);
+
+namespace Shlinkio\Shlink\Core\Exception;
+
+use Shlinkio\Shlink\Common\Exception\InvalidArgumentException;
+
+class NonUniqueSlugException extends InvalidArgumentException
+{
+    public static function fromSlug(string $slug): self
+    {
+        return new self(sprintf('Provided slug "%s" is not unique.', $slug));
+    }
+}