go.tools/dashboard: include builder in "invalid key" error message
go.tools/dashboard: include builder in "invalid key" error message

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/77250043

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