Move validation of datacatalogs/sets/downloads from beng-lod-server to lod-importer

[mwigham]

The idea is that the beng-lod-server should only need to deliver what is in the triple store, without any checks or validation. Validation should occur at the point at which import is done. Otherwise we can have the situation that invalid data is present in the triple store, where it is accessible via the endpoint while the beng-lod-server will block it, creating disparities.

NB: validation of the datacatalog/set/download ID should still be done in the beng-lod-server, as that handles errors on the request side rather than on the data side.

[wmelder]

I guess the issues on the spreadsheet dataset-importer show that validation on import is now realized. Validation on the beng-lod-server side is still in place in the datacatalog handler, so we might still consider to remove that, because it is duplicate.
But, it is not urgent.