[sources] Skip problematic sources when starting shell instead of crashing. Closes #3670208 At least this gives an opportunity to fix the problem in using c-c shell, which is currently not possible because it crashes during initialization. Notice that since sources are copy based, it should not be a problem at all to have some disabled, beside for services such as authentication sources as ldapfeed.

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