backends: make NewObject return fs.ErrorIsDir if possible

This changes the interface to NewObject so that if NewObject is called
on a directory then it should return fs.ErrorIsDir if possible without
doing any extra work, otherwise fs.ErrorObjectNotFound.

Tested on integration test server with:

go run integration-test.go -tests backend -run TestIntegration/FsMkdir/FsPutFiles/FsNewObjectDir -branch fix-stat -maxtries 1

backend/mailru/mailru.go

@@ -1958,7 +1958,7 @@ func (o *Object) readMetaData(ctx context.Context, force bool) error {
 	}
 	newObj, ok := entry.(*Object)
 	if !ok || dirSize >= 0 {
-		return fs.ErrorNotAFile
+		return fs.ErrorIsDir
 	}
 	if newObj.remote != o.remote {
 		return fmt.Errorf("File %q path has changed to %q", o.remote, newObj.remote)