templates/500.html

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8" />
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <title>500 Internal server error</title>
        <link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css" rel="stylesheet">
    </head>
    <body>
        <div class="container-fluid">
            <div class="row">
                <div class="col-md-8">
                    <div class="page-header">
                        <h1>500 Internal server error</h1>
                    </div>
                    <div class="alert alert-danger">
                        Something went terribly wrong and we were not able to process your request. Sorry.
                    </div>
                </div>
            </div>
        </div>
    </body>
</html>