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/filefabric/filefabric.go

@@ -1089,7 +1089,7 @@ func (o *Object) Size() int64 {
 // setMetaData sets the metadata from info
 func (o *Object) setMetaData(info *api.Item) (err error) {
 	if info.Type != api.ItemTypeFile {
-		return errors.Wrapf(fs.ErrorNotAFile, "%q is %q", o.remote, info.Type)
+		return fs.ErrorIsDir
 	}
 	o.hasMetaData = true
 	o.size = info.Size