fix: use templater to substitute values in command string
bytes.format() isn't supported on Python 3. Luckily, our template syntax
is similar so we can reuse it. We need a hack to disable \-escapes as '\'
is a directory separator on Windows.
{header}
<title>Mercurial Error</title>
</head>
<body>
<h2>Mercurial Error</h2>
<p>
An error occurred while processing your request:
</p>
<p>
{error|escape}
</p>
{footer}