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

TBR=bradfitz
R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/152250043

karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools