extensions: factor import error reporting out
To clarify third party extensions lookup, we are about to add a third place
where extensions are searched for. So we factor the error reporting logic out to
be able to easily reuse it in the next patch.
{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}