[errors] de-specialize Transient in favor of Tags.

grpc/prpc/encoding.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
 
 // This file implements encoding of RPC results to HTTP responses.
 
 import (
         "bytes"
@@ skipping 70 matching lines @@
                         _, err = buf.WriteRune('\n')
                 }
                 res.body = buf.Bytes()
 
         case FormatText:
                 var buf bytes.Buffer
                 err = proto.MarshalText(&buf, msg)
                 res.body = buf.Bytes()
 
         default:
-                panic(fmt.Errorf("impossible: invalid format %s", format))
+                panic(fmt.Errorf("impossible: invalid format %d", format))
 
         }
         if err != nil {
                 return errResponse(codes.Internal, 0, escapeFmt(err.Error()))
         }
 
         return &res
 }
 
 // respondProtocolError creates a response for a pRPC protocol error.
@@ skipping 36 matching lines @@
 }
 
 // codeStatus maps gRPC codes to HTTP status codes.
 // Falls back to http.StatusInternalServerError.
 func codeStatus(code codes.Code) int {
         if status, ok := codeToStatus[code]; ok {
                 return status
         }
         return http.StatusInternalServerError
 }