Fix IndexError when ifup does not give error msg

There are some rare, but existing cases in which ifup does not print
any error messages.

Change-Id: I072375b03cf76d86e455ea965a96d1871325d25a
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13260
Reviewed-by: Dan Kenigsberg <danken@redhat.com>