[errors] Greatly simplify common/errors package.

luci_config/server/cfgclient/backend/format/formatters.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 format
 
 import (
         "fmt"
         "sync"
 
@@ skipping 46 matching lines @@
         registry.r = nil
 }
 
 // getFormatter returns the Formatter associated with the provided Format.
 func getFormatter(f string) (Formatter, error) {
         registry.RLock()
         defer registry.RUnlock()
 
         formatter := registry.r[f]
         if formatter == nil {
-                return nil, errors.Reason("unknown formatter: %(formatter)q").D("formatter", f).Err()
+                return nil, errors.Reason("unknown formatter: %q", f).Err()
         }
         return formatter, nil
 }