[errors] Greatly simplify common/errors package.

vpython/spec/spec.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 spec
 
 import (
         "crypto/sha256"
         "encoding/hex"
         "fmt"
@@ skipping 52 matching lines @@
                 pos++
         }
         spec.Wheel = spec.Wheel[:pos]
 
         sort.Sort(specPackageSlice(spec.Wheel))
 
         // No duplicate packages. Since we're sorted, we can just check for no
         // immediate repetitions.
         for i, pkg := range spec.Wheel {
                 if i > 0 && pkg.Name == spec.Wheel[i-1].Name {
-                        return errors.Reason("duplicate spec entries for package %(path)q").
-                                D("name", pkg.Name).
-                                Err()
+                        return errors.Reason("duplicate spec entries for package %q", pkg.Name).Err()
                 }
         }
 
         return nil
 }
 
 // Hash hashes the contents of the supplied "spec" and "rt" and returns the
 // result as a hex-encoded string.
 //
 // If not empty, the contents of extra are prefixed to hash string. This can
@@ skipping 43 matching lines @@
 func (s pep425TagSlice) Len() int      { return len(s) }
 func (s pep425TagSlice) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
 
 func (s pep425TagSlice) Less(i, j int) bool {
         return sortby.Chain{
                 func(i, j int) bool { return s[i].Python < s[j].Python },
                 func(i, j int) bool { return s[i].Abi < s[j].Abi },
                 func(i, j int) bool { return s[i].Platform < s[j].Platform },
         }.Use(i, j)
 }