purge: prevent a silly crash with --confirm --files
if --files is passed, there was no directory to checks and `msg` was undefined.
This is now fixed and tested.
{header}
<title>Mercurial repository not found</title>
</head>
<body>
<h2>Mercurial repository not found</h2>
The specified repository "{repo|escape}" is unknown, sorry.
Please go back to the <a href="{url|urlescape}">main repository list page</a>.
{footer}