menu.lua: Abort autoboot sequence on failed command

Currently, a timeout in the menu autoboot sequence would effectively do
nothing. We would return from the autoboot handling, then begin processing
the menu without redrawing it.

This change makes the behavior a little more friendly. Returning the user to
the menu can't have any good effects, so abort the autoboot sequence and
drop to the loader prompt.

MFC after: 3 days