text/template turned this into an error but html/template crashed.
Refactor text/template.Execute to export a new function,
text/template.DefinedTemplates, so html/template can get the same
helpful error message in this case, and invoke it when there is no
definition for a template being escaped.

Fixes #10204.

Change-Id: I1d04e9e7ebca829bc08509caeb65e75da969711f
Reviewed-on: https://go-review.googlesource.com/7855Reviewed-by: Russ Cox <rsc@golang.org>