text/template: improve the error reporting for unexported fields. Changes suggested by rsc after last CL.
text/template: improve the error reporting for unexported fields.
Changes suggested by rsc after last CL.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6117044

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