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/local/local.go

@@ -401,7 +401,7 @@ func (f *Fs) newObjectWithInfo(remote string, info os.FileInfo) (fs.Object, erro
 
 	}
 	if o.mode.IsDir() {
-		return nil, errors.Wrapf(fs.ErrorNotAFile, "%q", remote)
+		return nil, fs.ErrorIsDir
 	}
 	return o, nil
 }