[req.find] Use vreg.schema.eschema for eschema lookup If an etype is non-existant, using vreg.schema[etype] raises a confusing error complaining about a non-existing relation. This is because of the implementation of vreg.schema.__getitem__ that look first in entities and then in relations. Using directly vreg.schema.eschema restrict the lookup to etypes only, hence raises a meaningful error when the etype does not exist.

raise NotImplementedError("Cannot migrate such an old version. Use intermediate Cubiweb version (try 3.16.x)")