run-test: clarify the error with a bad --with-hg is passed
This helped me to understand what was going on when I got into trouble.
Differential Revision: https://phab.mercurial-scm.org/D11046
{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}