godoc: be quiet by default about bad examples
godoc: be quiet by default about bad examples

Fixes golang/go#6759

R=golang-dev, adg, rsc
CC=golang-dev
https://golang.org/cl/26420044

4ad/tools,4ad/tools,golang/tools,nicot/tools,cw2018/tools,glasser/golang-tools,argandas/tools,cw2018/tools,jdhenke/tools,syreclabs/go-tools,magiccao/tools,runcom/tools,sshetty10/tools,golang/tools,glasser/golang-tools,glasser/golang-tools,syreclabs/go-tools,syreclabs/go-tools,cw2018/tools,cw2018/tools,magiccao/tools,dmitris/tools,maddyonline/tools,cmars/tools,syreclabs/go-tools,syreclabs/go-tools,cmars/tools,josharian/tools,argandas/tools,syreclabs/go-tools,sshetty10/tools,dmitris/tools,magiccao/tools,josharian/tools,magiccao/tools,golang/tools,cmars/tools,matryer/tools,matryer/tools,runcom/tools,4ad/tools,argandas/tools,josharian/tools,maddyonline/tools,jdhenke/tools,glasser/golang-tools,springning/tools,maddyonline/tools,runcom/tools,nicot/tools,argandas/tools,springning/tools,dmitris/tools,jdhenke/tools,springning/tools,springning/tools,golang/tools,sshetty10/tools,nicot/tools,maddyonline/tools,nicot/tools,josharian/tools,dmitris/tools,matryer/tools,sshetty10/tools,golang/tools,runcom/tools,cmars/tools,matryer/tools,jdhenke/tools