[errors] Greatly simplify common/errors package.

vpython/application/subcommand_delete.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 52 matching lines @@
                         }
 
                         logging.Infof(c, "Deleted VirtualEnv: %s", e.Name)
                         return nil
                 })
                 if err != nil {
                         return err
                 }
 
                 if failures > 0 {
-                        return errors.Reason("failed to delete %(count)d environment(s)").
-                                D("count", failures).
-                                Err()
+                        return errors.Reason("failed to delete %d environment(s)", failures).Err()
                 }
                 return nil
         })
 }