Add HTTP error annotation.

common/errors/tags.go

 // Copyright 2017 The LUCI Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //      http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 20 matching lines @@
         // applied to an error directly with the Apply method.
         //
         // Usually tag implementations will have a typesafe With method that generates
         // these. Avoid constructing these ad-hoc so that a given tag definition can
         // control the type safety around these.
         TagValue struct {
                 Key   TagKey
                 Value interface{}
         }
 
-        // A TagValueGenerator generates (TagKey, value) pairs, for use with Annoatator.Tag
+        // TagValueGenerator generates (TagKey, value) pairs, for use with Annoatator.Tag
         // and New().
         TagValueGenerator interface {
                 GenerateErrorTagValue() TagValue
         }
 )
 
 var _ sort.Interface = tagKeySlice(nil)
 
 func (s tagKeySlice) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
 func (s tagKeySlice) Len() int           { return len(s) }
@@ skipping 109 matching lines @@
                         for k, v := range ctx.tags {
                                 if _, ok := ret[k]; !ok {
                                         ret[k] = v
                                 }
                         }
                 }
                 return true
         })
         return ret
 }