[release-branch.go1.16] cmd/go: clarify errors for commands run outside a module

The new error message tells the user what was wrong (no go.mod found)
and directs them to 'go help modules', which links to tutorials.

Includes test fix from CL 298794
Fixes #44746

Change-Id: I98f31fec4a8757eb1792b45491519da4c552cb0f
Reviewed-on: https://go-review.googlesource.com/c/go/+/298650


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit b87e9b9f)
Reviewed-on: https://go-review.googlesource.com/c/go/+/298929