[errors] Greatly simplify common/errors package.

lucictx/lucictx.go

 // Copyright 2016 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 lucictx implements a Go client for the protocol defined here:
 //   https://github.com/luci/luci-py/blob/master/client/LUCI_CONTEXT.md
 //
 // It differs from the python client in a couple ways:
 //   * The initial LUCI_CONTEXT value is captured once at application start, and
 //     the environment variable is REMOVED.
@@ skipping 157 matching lines @@
                 return &nullExport{}, nil
         }
 
         if dir == "" {
                 dir = os.TempDir()
         }
         // Note: this makes a file in 0600 mode. This is what we want, the context
         // may have secrets.
         f, err := ioutil.TempFile(dir, "luci_context.")
         if err != nil {
-                return nil, errors.Annotate(err).Reason("creating luci_context file").Err()
+                return nil, errors.Annotate(err, "creating luci_context file").Err()
         }
 
         l := &liveExport{path: f.Name()}
         data, _ := json.Marshal(cur)
         _, err = f.Write(data)
         f.Close() // intentionally do this even on error.
         if err != nil {
                 l.Close() // cleans up the tempfile
-                return nil, errors.Annotate(err).Reason("writing luci_context").Err()
+                return nil, errors.Annotate(err, "writing luci_context").Err()
         }
         return l, nil
 }