ErrorContext now has all the information it needs from the Node,
rather than depending on the template that contains it. This makes
it easier for html/template to generate correct locations in its
error messages.

Updated html/template to use this ability where it is easy, which is
not everywhere, but more work can probably push it through.

Fixes #8577.

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/130620043