[errors] Greatly simplify common/errors package.

logdog/client/cmd/logdog_butler/streamserver.go

 // Copyright 2015 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 main
 
 import (
         "flag"
         "fmt"
         "strings"
@@ skipping 65 matching lines @@
 
         // Common implementations.
         switch typ {
         case "tcp4":
                 return streamserver.NewTCP4Server(ctx, spec)
 
         case "tcp6":
                 return streamserver.NewTCP6Server(ctx, spec)
 
         default:
-                return nil, errors.Reason("unknown stream server type: %(type)q").
-                        D("type", typ).
-                        Err()
+                return nil, errors.Reason("unknown stream server type: %q", typ).Err()
         }
 }