lib/deploy: Throw an error if trying to stage when not ostree-booted

lib/deploy: Throw an error if trying to stage when not ostree-booted

There's no reason to do this.  I didn't actually hit this problem,
but it's a corner case that just occurred to me while working on
the code.

I think callers should be adapted to skip trying to use staging
if there's no booted deployment.

Closes: #1568
Approved by: jlebon