smb: fix spurious "Directory already exists" errors when moving directories - fixes #9680

DirMove returned fs.ErrorDirExists for any error from the destination
existence check not just when the destination actually exists. That
made sync silently fall back to file-by-file moves and masked the real
failure.

Return the underlying error instead when the check fails for any other
reason.

backend/smb/smb.go

@@ -439,11 +439,13 @@ func (f *Fs) DirMove(ctx context.Context, src fs.Fs, srcRemote, dstRemote string
 	defer f.putConnection(&cn, err)
 
 	_, err = cn.smbShare.Stat(f.toSambaPath(dstPath))
-	if os.IsNotExist(err) {
-		err = cn.smbShare.Rename(f.toSambaPath(srcPath), f.toSambaPath(dstPath))
-		return translateError(err, true)
+	if err == nil {
+		return fs.ErrorDirExists
+	} else if !os.IsNotExist(err) {
+		return fmt.Errorf("failed to check destination directory: %w", err)
 	}
-	return fs.ErrorDirExists
+	err = cn.smbShare.Rename(f.toSambaPath(srcPath), f.toSambaPath(dstPath))
+	return translateError(err, true)
 }
 
 // List files and directories in a directory