This moots a caveat in the proposed package documentation by
rendering useless any template that could not be escaped.

From https://golang.org/cl/4969078/
> If EscapeSet returns an error, do not Execute the set; it is not
> safe against injection.
r: [but isn't the returned set nil? i guess you don't overwrite the
r: original if there's a problem, but i think you're in your rights to
r: do so]

R=r
CC=golang-dev
https://golang.org/cl/5020043