cmd/vet: don't complain about Error() Fixes issue 4598.
cmd/vet: don't complain about Error()
Fixes issue 4598.

R=golang-dev, dsymonds, r
CC=golang-dev
https://codereview.appspot.com/7102050

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