[PATCH 4 of 4 V2] templater: add symbol to error

[Sean Farley]

Ryan McElroy writes:

> # HG changeset patch
> # User Ryan McElroy <rmcelroy at fb.com>
> # Date 1402621641 25200
> #      Thu Jun 12 18:07:21 2014 -0700
> # Node ID fb64043de7c4ffc79ad3639d2061e1861522c084
> # Parent  ef4d286cea6073a99bc1b6196dc35627ee89ad8e
> templater: add symbol to error
>
> This patch makes it easier to debug writing templater functions by
> telling the user exactly what was sent to the templater.

These look good to me.