[errors] Greatly simplify common/errors package.

vpython/application/subcommand_install.go

 // Copyright 2017 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package application
 
 import (
         "github.com/maruel/subcommands"
         "golang.org/x/net/context"
 
@@ skipping 32 matching lines @@
         c := cli.GetContext(app, cr, env)
         a := getApplication(c, args)
         a.opts.EnvConfig.PruneThreshold = 0 // Don't prune on install.
         a.opts.EnvConfig.OverrideName = cr.name
 
         return run(c, func(c context.Context) error {
                 err := venv.With(c, a.opts.EnvConfig, false, func(context.Context, *venv.Env) error {
                         return nil
                 })
                 if err != nil {
-                        return errors.Annotate(err).Reason("failed to setup the environment").Err()
+                        return errors.Annotate(err, "failed to setup the environment").Err()
                 }
 
                 logging.Infof(c, "Successfully setup the environment.")
                 return nil
         })
 }