[errors] Greatly simplify common/errors package.

common/system/filesystem/filesystem_test.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 filesystem
 
 import (
         "io/ioutil"
         "os"
         "path/filepath"
@@ skipping 35 matching lines @@
                 _, err := os.Open(dnePath)
                 if !os.IsNotExist(err) {
                         t.Fatalf("failed to get IsNotExist error: %s", err)
                 }
 
                 Convey(`IsNotExist works`, t, func() {
                         So(IsNotExist(nil), ShouldBeFalse)
                         So(IsNotExist(errors.New("something")), ShouldBeFalse)
 
                         So(IsNotExist(err), ShouldBeTrue)
-                        So(IsNotExist(errors.Annotate(err).Reason("annotated").Err()), ShouldBeTrue)
+                        So(IsNotExist(errors.Annotate(err, "annotated").Err()), ShouldBeTrue)
                 })
         })
 }
 
 func TestAbsPath(t *testing.T) {
         t.Parallel()
 
         // Create a temporary directory so we control its contents. This will let us
         // safely create an IsNotExist error.
         withTempDir(t, func(tdir string) {
@@ skipping 168 matching lines @@
                                         So(isGone(subDir), ShouldBeTrue)
                                 })
                         })
 
                         Convey(`Will return nil if the target does not exist`, func() {
                                 So(RemoveAll(filepath.Join(tdir, "dne")), ShouldBeNil)
                         })
                 })
         })
 }