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

skyportsystems/go.tools,d0f/go-zh.tools,8limbs/gotools,webfd/go-zh.tools,8limbs/gotools,d0f/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,8limbs/gotools,bryanxu/go-zh.tools