Print an error when attempting to use bind mounts without chroot.

Bind mounts should be used with chroot or pivot_root. Print an error
and exit when that's not the case.

Clean up some comments and error messages while in there.

Bug: 26784268
Change-Id: I4e384a989e1aef5b2989c4f17e047a9ac7cadbc8
2 files changed