[errors] Greatly simplify common/errors package.

lucictx/lucictx.go

Index: lucictx/lucictx.go
diff --git a/lucictx/lucictx.go b/lucictx/lucictx.go
index dd9bb7b866dca2699d15724666fb63f4a5ffae16..d42cd9164b5fe0853f7ecfe7e65a9ec02c984d0f 100644
--- a/lucictx/lucictx.go
+++ b/lucictx/lucictx.go
@@ -175,7 +175,7 @@ func Export(ctx context.Context, dir string) (Exported, error) {
 	// 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()}
@@ -184,7 +184,7 @@ func Export(ctx context.Context, dir string) (Exported, error) {
 	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
 }