Error pages

Looking at a lot of VCL example, this seems to be a standard pattern.

Of all the other options, this wins on simplicity.

1. If a backend throws an error we catch it in vcl_fetch and pass it to vcl_error
2. vcl_error generates a synthetic page.
3. The page points at the heroku error app (via an iframe).

/cc @wheresrhys 
I need to inject the `obj.status` in the src URL.