go/*: Add Unwrap to custom errors that have Cause

So that Go1.13 and golang.org/x/xerrors understand the wrapping:
https://blog.golang.org/go1.13-errors

Preserve Cause for compatibility with github.com/pkg/errors.

go/neo/neonet/connection.go

-// Copyright (C) 2016-2018  Nexedi SA and Contributors.
+// Copyright (C) 2016-2020  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -1281,8 +1281,10 @@ func (e *ConnError) Error() string {
 	return fmt.Sprintf("%s .%d: %s: %s", e.Link, e.ConnId, e.Op, e.Err)
 }
 
-func (e *LinkError) Cause() error { return e.Err }
-func (e *ConnError) Cause() error { return e.Err }
+func (e *LinkError) Cause()  error { return e.Err }
+func (e *LinkError) Unwrap() error { return e.Err }
+func (e *ConnError) Cause()  error { return e.Err }
+func (e *ConnError) Unwrap() error { return e.Err }
 
 func (nl *NodeLink) err(op string, e error) error {
 	if e == nil {

go/zodb/storage/fs1/format.go

-// Copyright (C) 2017-2019  Nexedi SA and Contributors.
+// Copyright (C) 2017-2020  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -96,9 +96,8 @@ type RecordError struct {
 	Err    error  // actual error
 }
 
-func (e *RecordError) Cause() error {
-	return e.Err
-}
+func (e *RecordError) Cause() error  { return e.Err }
+func (e *RecordError) Unwrap() error { return e.Err }
 
 func (e *RecordError) Error() string {
 	// XXX omit path: when .Err already contains it (e.g. when it is os.PathError)?

go/zodb/zodb.go

-// Copyright (C) 2016-2019  Nexedi SA and Contributors.
+// Copyright (C) 2016-2020  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -315,9 +315,8 @@ func (e *OpError) Error() string {
 	return s
 }
 
-func (e *OpError) Cause() error {
-	return e.Err
-}
+func (e *OpError) Cause() error  { return e.Err }
+func (e *OpError) Unwrap() error { return e.Err }
 
 
 // IStorage is the interface provided by opened ZODB storage.