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/putio/object.go

@@ -151,7 +151,7 @@ func (o *Object) readEntry(ctx context.Context) (f *putio.File, err error) {
 		return nil, err
 	}
 	if resp.File.IsDir() {
-		return nil, fs.ErrorNotAFile
+		return nil, fs.ErrorIsDir
 	}
 	return &resp.File, err
 }