cmd/go: report non-Go files as package error

This change modifies cmd/go/list to format the error correctly in case
-e flag is set. It also fixes a bug where the package loader was only
ever checking the first pattern if it had the go extension. This caused
and error when a file without .go extension was not the first argument.

Fixes #29899

Change-Id: I029bf4465ad4ad054434b8337c1d2a59369783da
Reviewed-on: https://go-review.googlesource.com/c/go/+/166398


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>