Silence `tc` output when `ignore_fails` is true

[RalfJung]

The traffic_control code uses the tc command to interact with the Linux kernel traffic shaping infrastructure. The way we run it leads to harmless but confusing errors being printed in the log (see e.g. https://github.com/wlanslovenija/tunneldigger/issues/167). It would be better to suppress tc output when errors are harmless (as indicated by the ignore_fails flag).