[errors] Greatly simplify common/errors package.

grpc/grpcutil/errors.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 grpcutil
 
 import (
         "google.golang.org/grpc"
         "google.golang.org/grpc/codes"
 
@@ skipping 71 matching lines @@
         }
 
         if IsTransientCode(Code(err)) {
                 err = transient.Tag.Apply(err)
         }
         return err
 }
 
 type grpcCodeTag struct{ Key errors.TagKey }
 
-func (g grpcCodeTag) With(code codes.Code) errors.TagValue { return errors.MkTagValue(g.Key, code) }
+func (g grpcCodeTag) With(code codes.Code) errors.TagValue {
+        return errors.TagValue{Key: g.Key, Value: code}
+}
 func (g grpcCodeTag) In(err error) (v codes.Code, ok bool) {
         d, ok := errors.TagValueIn(g.Key, err)
         if ok {
                 v = d.(codes.Code)
         }
         return
 }
 
 // Tag may be used to associate a gRPC status code with this error.
 //
@@ skipping 20 matching lines @@
 // transient gRPC error type.
 func IsTransientCode(code codes.Code) bool {
         switch code {
         case codes.Internal, codes.Unknown, codes.Unavailable:
                 return true
 
         default:
                 return false
         }
 }