go.tools/dashboard/app: document special error string
go.tools/dashboard/app: document special error string

TBR=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/152250043

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