An unmatched {{else}} should trigger a parsing error.

The top level parser is able to issue an error in case
of unmatched {{end}}. It does it a posteriori (i.e. after having
parsed the action).

Extend this behavior to also check for unmatched {{else}}

Fixes #10611

Change-Id: I1d4f433cc64e11bea5f4d61419ccc707ac01bb1d
Reviewed-on: https://go-review.googlesource.com/9620Reviewed-by: Rob Pike <r@golang.org>