1) The code to catch an exception marked the template as escaped
when it was not yet, which caused subsequent executions of the
template to not escape properly.
2) ensurePipelineContains needs to handled Field as well as
Identifier nodes.

Fixes #7379.

LGTM=mikesamuel
R=mikesamuel
CC=golang-codereviews
https://golang.org/cl/85240043