cmd/gc: re-word some error messages Fixes issue 6557.
cmd/gc: re-word some error messages
Fixes issue 6557.

R=golang-dev, rsc, tracey.brendan
CC=golang-dev
https://codereview.appspot.com/14432053

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