go/zodb: Fix thinko in NoObjectError.Error

This error type is documented (see Loader) to be always created as
*NoObjectError.

-> Fix Error receiver accordingly.

go/zodb/zodb.go

@@ -275,7 +275,7 @@ type NoObjectError struct {
 	Oid Oid
 }
 
-func (e NoObjectError) Error() string {
+func (e *NoObjectError) Error() string {
 	return fmt.Sprintf("%s: no such object", e.Oid)
 }