go.tools/dashboard/app: send log hash to gobot on build failure
go.tools/dashboard/app: send log hash to gobot on build failure

Fixes issue 7322.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/75630043

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