fmt: add Errorf helper function
fmt: add Errorf helper function

This crops up in a lot of places.
It's just a one-liner, but doesn't add any dependancies.
Seems worth it.

R=r, r2
CC=golang-dev
http://codereview.appspot.com/2344041

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