text/template: allow newlines inside action delimiters

This allows multiline constructs like:

	{{"hello" |
	  printf}}

Now that unclosed actions can span multiple lines,
track and report the start of the action when reporting errors.

Also clean up a few "unexpected <error message>" to be just "<error message>".

Fixes #29770.

Change-Id: I54c6c016029a8328b7902a4b6d85eab713ec3285
Reviewed-on: https://go-review.googlesource.com/c/go/+/254257


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>