[cwconfig] Issue a warning when a "core" ctl plugin failed to load This is often because of a missing dependency, the warning would indicate this hopefully.

etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances