Display argument list in a better way

The mail.html.twig file tries to json_encode any arguments which are passed to the methods in the stack trace, but if these arguments have any private properties (like doctrine objects) it will cause a 'recursion detected' warning. See this blog post for more info - http://blog.jezmckean.com/php-bug-json_encode-misleading-warning-on-object-with-private-properties/

I've changed the mail twig template so that it's more like Symfony's own formatter and calls format_args instead of json_encode, which fixes the problem.