[validation api] properly use yams 0.36 validation error api and update message catalog. Follows bbe0d6985e59 the creation of the `translate` method in the 23a10f049447 yams commit.

[wsgi] saner use of `self.config` instead of `config`

[server] fix repository initialisation Changeset d753d6a6798f was breaking database creation. Dropping the `config.creating` clause lead to trying to load the schema from database even in creation mode. Conditional are a bit altered and reordered to prevent this to happen.

[web test] make unittest_viewselectors work if rdflib isn't available