tests: allow more detailed error message from re.compile
test-grep.t now passes on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D4240
{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}