cmd/gorelease: detect problems before releasing new module versions

gorelease is an experimental tool that helps module authors avoid
common problems before releasing a new version of a module.

gorelease is intended to eventually be merged into the go command as
"go release".

Updates golang/go#26420

Change-Id: I23fbab171b2cb9f4598fa525ecef5dcf006dc7c4
Reviewed-on: https://go-review.googlesource.com/c/exp/+/197299
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: Jay Conrod

cmd/gorelease/errors.go

@@ -0,0 +1,52 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os/exec"
+	"strings"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/xerrors"
+)
+
+type usageError struct {
+	err error
+}
+
+func usageErrorf(format string, args ...interface{}) error {
+	return &usageError{err: fmt.Errorf(format, args...)}
+}
+
+const usageText = `usage: gorelease -base=version [-version=version]`
+
+func (e *usageError) Error() string {
+	msg := ""
+	if !xerrors.Is(e.err, flag.ErrHelp) {
+		msg = e.err.Error()
+	}
+	return usageText + "\n" + msg + "\nFor more information, run go doc golang.org/x/exp/cmd/gorelease"
+}
+
+type downloadError struct {
+	m   module.Version
+	err error
+}
+
+func (e *downloadError) Error() string {
+	var msg string
+	if xerr, ok := e.err.(*exec.ExitError); ok {
+		msg = strings.TrimSpace(string(xerr.Stderr))
+	} else {
+		msg = e.err.Error()
+	}
+	sep := " "
+	if strings.Contains(msg, "\n") {
+		sep = "\n"
+	}
+	return fmt.Sprintf("error downloading module %s@%s:%s%s", e.m.Path, e.m.Version, sep, msg)
+}