[repo] Don't crash on start when fs schema is missing some db schema entities
This occurs usually while developping and we don't want systematically to
rebuild the database to start the instance or run e.g. i18ninstance.
usr/lib/python2*/*-packages/cubicweb/web
usr/lib/python2*/*-packages/cubicweb/wsgi
usr/share/cubicweb/cubes/shared/data
usr/share/cubicweb/cubes/shared/wdoc