bectl: Improve error message when ZFS root is not found.

When recovering a system that is unbootable due to some
problem with the active BE, it is likely you'll be booted
from a rescue image running UFS. In this case, bectl
needs help finding the zpool root that you want to operate
on. In this case, improve the error message to suggest
specifying a root, rather than just emitting a generic
error message that might imply, to the naive user, that
there is a ZFS compatibility issue between the rescue
image and the on-disk ZFS pool.

Reviewed by: imp, kevans
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D39346