It is not important to add, since it's only used for creating an error message,
but for consistency in the API between text/template and html/template
it should be provided here.

The implementation just calls the one in text/template.

Fixes #13349.

Change-Id: I0882849e06a58f1e38b00eb89d79ac39777309b2
Reviewed-on: https://go-review.googlesource.com/17172Reviewed-by: Andrew Gerrand <adg@golang.org>