This is a followup to ed7b44370f71126087eb953f36aad8dc4c44109f;
following that commit, 'guix offload test' and 'guix offload status'
would abort with a backtrace instead of clearly diagnosing a missing
'guix' command on the build machine.
* guix/scripts/offload.scm (assert-node-has-guix): Call 'leave' when
NODE is not an inferior. Remove 'catch' blocks for 'node-repl-error'.
(check-machine-availability): Invoke 'assert-node-has-guix' first.
(check-machine-status): Print a warning when 'remote-inferior' returns #f.