[web/httpcache] send weak ETags
There's no way we can provide a strong validator in general (especially
considering how ETags are generated by the current EtagHTTPCacheManager
and EntityHTTPCacheManager). So mark our entity-tags as weak.
[server] print backend exception if we fail to execute an sql script
It's more useful to see
could not open extension control file "/usr/share/postgresql/9.4/extension/postgis.control": No such file or directory
rather than merely
-> ERROR, skipping /usr/share/cubicweb/cubes/postgis/schema/_ext.postgres.sql