[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.
missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/share/pyshared/cubicweb/skeleton/debian/rules.tmpl)