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://golang.org/cl/13146043

argandas/tools,josharian/tools,cmars/tools,runcom/tools,golang/tools,matryer/tools,dmitris/tools,maddyonline/tools,runcom/tools,4ad/tools,dmitris/tools,matryer/tools,cw2018/tools,jdhenke/tools,glasser/golang-tools,springning/tools,matryer/tools,cmars/tools,magiccao/tools,cmars/tools,jdhenke/tools,sshetty10/tools,josharian/tools,dmitris/tools,syreclabs/go-tools,golang/tools,jdhenke/tools,golang/tools,josharian/tools,magiccao/tools,golang/tools,matryer/tools,cw2018/tools,nicot/tools,argandas/tools,glasser/golang-tools,runcom/tools,magiccao/tools,nicot/tools,syreclabs/go-tools,springning/tools,nicot/tools,cmars/tools,maddyonline/tools,argandas/tools,springning/tools,maddyonline/tools,sshetty10/tools,springning/tools,maddyonline/tools,syreclabs/go-tools,nicot/tools,magiccao/tools,jdhenke/tools,4ad/tools,glasser/golang-tools,glasser/golang-tools,syreclabs/go-tools,golang/tools,sshetty10/tools,josharian/tools,dmitris/tools,4ad/tools,runcom/tools,cw2018/tools,syreclabs/go-tools,sshetty10/tools,syreclabs/go-tools,argandas/tools,cw2018/tools