exp/template: improve error message for non-exported field.
exp/template: improve error message for non-exported field.

R=golang-dev, dsymonds
CC=golang-dev
http://codereview.appspot.com/4636086

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang