Throw error if tc interface retrieval fails (#56)
Throw error if tc interface retrieval fails (#56)

* Throw error if tc interface retrieval fails

On Linux if `net-tools` are not installed `tc` will fail to get the default interface using the `route` command. Or it might fail for any other reason.

This commit adds a safeguard to bug out on err and displays the actual stderr received instead of hiding it.

sitespeedio/throttle,sitespeedio/throttle