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

apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,apparentlymart/golang