go/vcs: add proper error handling to test.
go/vcs: add proper error handling to test.

Fix a couple of error messages to report better messages.

R=adg
CC=golang-dev
https://codereview.appspot.com/13146043

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