Exception message when trying to print an object without __toString() in a Twig template is not helpful

@Berdir mentioned this issue to me today on IRC.

Relevant code from twig.engine:

  if (is_object($arg)) {
    if (method_exists($arg, '__toString')) {
      return (string) $arg;
    }
    throw new Exception(t('Object of type "@class" cannot be printed.', array('@class' => get_class($arg))));
  }

If possible we should mention the variable name as well as the template where this is being called from - that would be a big improvement.