Check if SortIndex is a number when saving Data

[Timo-Breumelhof]

To prevent this issue:

https://opencontent.readme.io/docs/sorting#lucene-issues

It would be nice if OC would check the saved data for the SortIndex field and raise an error if it's not a number. An error and the reason in the logs would be fine. As discussed before we should try to prevent a tiny mistake in the schema file to mess up the (Open Content internal) lucene index.