[errors] Greatly simplify common/errors package.

common/config/impl/filesystem/fs.go

 // Copyright 2015 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 filesystem implements a file system backend for the config client.
 //
 // May be useful during local development.
 //
 // Layout
 //
@@ skipping 167 matching lines @@
                 scannedConfigs:          newScannedConfigs(),
                 contentRevisionsScanned: stringset.New(1),
         }
 
         if ret.islink {
                 inf, err := os.Stat(basePath)
                 if err != nil {
                         return nil, err
                 }
                 if !inf.IsDir() {
-                        return nil, (errors.Reason("filesystem.New(%(basePath)q): does not link to a directory").
-                                D("basePath", basePath).Err())
+                        return nil, errors.Reason("filesystem.New(%q): does not link to a directory", basePath).Err()
                 }
                 if len(ret.basePath.explode()) < 1 {
-                        return nil, (errors.Reason("filesystem.New(%(basePath)q): not enough tokens in path").
-                                D("basePath", basePath).Err())
+                        return nil, errors.Reason("filesystem.New(%q): not enough tokens in path", basePath).Err()
                 }
         } else if !inf.IsDir() {
-                return nil, (errors.Reason("filesystem.New(%(basePath)q): not a directory").
-                        D("basePath", basePath).Err())
+                return nil, errors.Reason("filesystem.New(%q): not a directory", basePath).Err()
         }
         return ret, nil
 }
 
 func (fs *filesystemImpl) resolveBasePath() (realPath nativePath, revision string, err error) {
         if fs.islink {
                 realPath, err = fs.basePath.readlink()
                 if err != nil && err.(*os.PathError).Err != os.ErrInvalid {
                         return
                 }
@@ skipping 323 matching lines @@
         ret := stringset.New(0)
         err := fs.iterContentRevPath(func(lk lookupKey, cfg *config.Config) {
                 if lk.configSet.hasPrefix(pfx) {
                         ret.Add(newConfigSet(lk.configSet.explode()[2:]...).s())
                 }
         })
         retSlc := ret.ToSlice()
         sort.Strings(retSlc)
         return retSlc, err
 }