go.tools/go/types: fix nil-ptr deref in gcimporter
go.tools/go/types: fix nil-ptr deref in gcimporter

Also: Make error handling more explicit.

Possibly a fix for issue 5758.

R=adonovan
CC=golang-dev
https://golang.org/cl/10520043

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