[errors] Greatly simplify common/errors package.

tokenserver/appengine/impl/certconfig/rpc_fetch_crl.go

 // Copyright 2016 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 certconfig
 
 import (
         "crypto/x509"
         "fmt"
         "io/ioutil"
@@ skipping 124 matching lines @@
 
         // Read the body in its entirety.
         blob, err = ioutil.ReadAll(resp.Body)
         if err != nil {
                 return nil, "", transient.Tag.Apply(err)
         }
 
         // Transient error?
         if resp.StatusCode >= http.StatusInternalServerError {
                 logging.Warningf(c, "GET %s - HTTP %d; %q", cfg.CrlUrl, resp.StatusCode, string(blob))
-                return nil, "", errors.Reason("server replied with HTTP %(code)d").
-                        D("code", resp.StatusCode).Tag(transient.Tag).Err()
+                return nil, "", errors.Reason("server replied with HTTP %d", resp.StatusCode).
+                        Tag(transient.Tag).Err()
         }
 
         // Something we don't support or expect?
         if resp.StatusCode != http.StatusOK {
                 logging.Errorf(c, "GET %s - HTTP %d; %q", cfg.CrlUrl, resp.StatusCode, string(blob))
                 return nil, "", fmt.Errorf("unexpected status HTTP %d", resp.StatusCode)
         }
 
         // Good enough.
         return blob, resp.Header.Get("ETag"), nil
@@ skipping 57 matching lines @@
                 }
                 return ds.Put(c, toPut)
         }, nil)
         if err != nil {
                 return nil, transient.Tag.Apply(err)
         }
 
         logging.Infof(c, "CRL for %q is updated, entity version is %d", ca.CN, updated.EntityVersion)
         return updated, nil
 }