WebClient: improve error message when trying to move non-empty folder

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

internal/vfs/gcsfs.go

@@ -19,6 +19,7 @@ package vfs
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"mime"
@@ -448,7 +449,7 @@ func (*GCSFs) IsNotSupported(err error) bool {
 	if err == nil {
 		return false
 	}
-	return err == ErrVfsUnsupported
+	return errors.Is(err, ErrVfsUnsupported)
 }
 
 // CheckRootPath creates the specified local root directory if it does not exists
@@ -774,7 +775,7 @@ func (fs *GCSFs) renameInternal(source, target string, fi os.FileInfo, recursion
 				return numFiles, filesSize, err
 			}
 			if hasContents {
-				return numFiles, filesSize, fmt.Errorf("cannot rename non empty directory: %q", source)
+				return numFiles, filesSize, fmt.Errorf("%w: cannot rename non empty directory: %q", ErrVfsUnsupported, source)
 			}
 		}
 		if err := fs.mkdirInternal(target); err != nil {