cmd/updatestd: print stderr in runOut when command fails

Stderr is much more likely to contain useful information than
stdout in the unexpected case of the command failing. Show it.

For golang/go#55067.

Change-Id: Ice85e957bb7d9104da475d7616519bbfbae105e3
Reviewed-on: https://go-review.googlesource.com/c/build/+/431357
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

cmd/updatestd/updatestd.go

@@ -12,6 +12,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"errors"
 	"flag"
 	"fmt"
 	"go/ast"
@@ -278,6 +279,9 @@ func (r runner) runOut(args ...string) []byte {
 	out, err := cmd.Output()
 	if err != nil {
 		log.Printf("> %s\n", strings.Join(args, " "))
+		if ee := (*exec.ExitError)(nil); errors.As(err, &ee) {
+			out = append(out, ee.Stderr...)
+		}
 		log.Fatalf("command failed: %s\n%s", err, out)
 	}
 	return out