template: use panic/recover to simplify internal error handling.
template: use panic/recover to simplify internal error handling.

R=rsc
CC=golang-dev
http://codereview.appspot.com/824049

wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old