The current parser ignores obvious errors such as:
{{0.1.E}}
{{true.any}}
{{"hello".wrong}}
{{nil.E}}

The common problem is that a chain is built from
a literal value. It then panics at execution time.

Furthermore, a double dot triggers the same behavior:
{{..E}}

Addresses a TODO left in Tree.operand to catch these
errors at parsing time.

Note that identifiers can include a '.', and pipelines
could return an object which a field can be derived
from (like a variable), so they are excluded from the check.

Fixes #10615

Change-Id: I903706d1c17861b5a8354632c291e73c9c0bc4e1
Reviewed-on: https://go-review.googlesource.com/9621Reviewed-by: Rob Pike <r@golang.org>