text/template: detect unexported fields better

text/template: detect unexported fields better
Moves the error detection back into execution, where it used to be,
and improves the error message.
Rolls back most of 6009048, which broke lower-case keys in maps.
If it weren't for maps we could detect this at compile time rather than
execution time.

Fixes #3542.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6098051