[errors] Greatly simplify common/errors package.

grpc/prpc/client.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 prpc
 
 import (
         "bytes"
         "fmt"
         "io"
@@ skipping 186 matching lines @@
                         req.Body = ioutil.NopCloser(bytes.NewReader(in))
                         res, err := ctxhttp.Do(ctx, c.getHTTPClient(), req)
                         if res != nil && res.Body != nil {
                                 defer res.Body.Close()
                         }
                         if c.testPostHTTP != nil {
                                 err = c.testPostHTTP(ctx, err)
                         }
                         if err != nil {
                                 // Treat all errors here as transient.
-                                return errors.Annotate(err).Reason("failed to send request").
-                                        Tag(transient.Tag).Err()
+                                return errors.Annotate(err, "failed to send request").Tag(transient.Tag).Err()
                         }
 
                         if options.resHeaderMetadata != nil {
                                 *options.resHeaderMetadata = metadataFromHeaders(res.Header)
                         }
                         contentType = res.Header.Get("Content-Type")
 
                         // Read the response body.
                         buf.Reset()
                         var body io.Reader = res.Body
@@ skipping 145 matching lines @@
         if len(h) == 0 {
                 return nil
         }
 
         md := make(metadata.MD, len(h))
         for k, v := range h {
                 md[strings.ToLower(k)] = v
         }
         return md
 }