Provide a slightly more informative panic when RootFs is called without a container
Provide a slightly more informative panic when RootFs is called without a container

Summary: Prior to this, it panicd within the C-interop layer of the LXC libs.

Test Plan: None

Reviewers: jukka

Reviewed By: jukka

Subscribers: changesbot, mkedia, anupc

Differential Revision: https://tails.corp.dropbox.com/D130352

dropbox/changes-client,dropbox/changes-client