go.tools/go/loader: add missing argument to fmt.Errorf call
go.tools/go/loader: add missing argument to fmt.Errorf call

R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/53360043

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old