tc_wrapper: Fix errno parsing for multiline error text

Sometimes tc appends an extra text after the parseable error. E.g.:

    RTNETLINK answers: Invalid argument\nWe have an error talking to the kernel\n

This patch makes it possible to handle such cases

Change-Id: Ica50f0be8ff842421933ec8bb441ddc6e5cc19bb
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31457
Reviewed-by: Dan Kenigsberg <danken@redhat.com>