go.tools/dashboard: update CL on build failure
go.tools/dashboard: update CL on build failure

Only send mail to golang-dev if updating the CL fails.

R=golang-codereviews, dvyukov, minux.ma, rsc
CC=golang-codereviews
https://codereview.appspot.com/43760043

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