It's a common error to reference unexported field names in templates,
especially for newcomers. This catches the error at parse time rather than
execute time so the rare few who check errors will notice right away.

These were always an error, so the net behavior is unchanged.
Should break no existing code, just identify the error earlier.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6009048