[errors] Greatly simplify common/errors package.

common/data/text/stringtemplate/template.go

 // Copyright 2017 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 stringtemplate implements Python string.Template-like substitution.
 package stringtemplate
 
 import (
         "regexp"
         "strings"
@@ skipping 42 matching lines @@
         var (
                 parts = make([]string, 0, (len(smi)*2)+1)
                 pos   = 0
         )
 
         for _, match := range smi {
                 key := ""
                 switch {
                 case match[8] >= 0:
                         // Invalid.
-                        return "", errors.Reason("invalid template: %(template)q").
-                                D("template", v).
-                                Err()
+                        return "", errors.Reason("invalid template: %q", v).Err()
 
                 case match[2] >= 0:
                         // Escaped.
                         parts = append(parts, v[pos:match[2]])
                         pos = match[3]
                         continue
 
                 case match[4] >= 0:
                         // Key (without braces)
                         key = v[match[4]:match[5]]
                 case match[6] >= 0:
                         // Key (with braces)
                         key = v[match[6]:match[7]]
 
                 default:
                         panic("impossible")
                 }
 
                 // Add anything in between the previous match and the current. If our match
                 // includes a non-escape character, add that too.
                 if pos < match[0] {
                         parts = append(parts, v[pos:match[0]])
                 }
                 pos = match[1]
 
                 subst, ok := subst[key]
                 if !ok {
-                        return "", errors.Reason("no substitution for %(key)q").
-                                D("key", key).
-                                Err()
+                        return "", errors.Reason("no substitution for %q", key).Err()
                 }
                 parts = append(parts, subst)
         }
 
         // Append any trailing string.
         parts = append(parts, v[pos:])
 
         // Join the parts.
         return strings.Join(parts, ""), nil
 }