cmd/go: replace formatOutput/showOutput with reportCmd

cmd/go: replace formatOutput/showOutput with reportCmd

The general pattern is to replace

	if len(cmdOut) > 0 {
		output := b.processOutput(cmdOut)
		if err != nil {
			err = formatOutput(b.WorkDir, dir, p.ImportPath, desc, output)
		} else {
			b.showOutput(a, dir, desc, output)
		}
	}
	if err != nil {
		return err
	}

with

	if err := b.reportCmd(a, p, desc, dir, cmdOut, err); err != nil {
		return err
	}

However, there is a fair amount of variation between call sites. The
most common non-trivial variation is sites where errors are an
expected outcome. In this case, often we simply pass "nil" for the
error to trigger only the printing behavior of reportCmd.

For #62067, but also a nice cleanup on its own.

Change-Id: Ie5f918017c02d8558f23ad4c38261077c0fa4ea3
Reviewed-on: https://go-review.googlesource.com/c/go/+/529219
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>