bootctl: return recognizable Varlink error when we cannot determine the boot entry...

bootctl: return recognizable Varlink error when we cannot determine the boot entry token

When running "bootctl install" on an empty --root= dir, we don't know
which token to use, and the operation will fail. Make sure to return an
explicit error about this.

This introduces a recognizable low-level error for this (EUNATCH), and
then turns this into a recognizable Varlink error.

(I made sure that the old low-level error EINVAL wasn't load-bearing,
and it is safe to change this.)