image/png: fix error message to not return width twice
image/png: fix error message to not return width twice

Fixes issue 5413

R=golang-dev, dave, adg
CC=golang-dev
https://codereview.appspot.com/9153045

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