cmd/go: add base.Error and base.Fatal

cmd/go: add base.Error and base.Fatal

Many many places in the go command use

	base.Errorf("go: %v", err)

or

	base.Fatalf("go: %v", err)

Introduce Error(error) and Fatal(error) to do this
and update all call sites (global search and replace).

The new Error gives us the opportunity to unwrap
a multierror and add the go prefix to each line,
which is the motivation for this change.
(We want to start returning a multierror from LoadModFile
and LoadModGraph.)

For #57001.

Change-Id: I9613653b94808224146077c30d22f814d4e19eed
Reviewed-on: https://go-review.googlesource.com/c/go/+/499980
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>