lualoader: disable autoboot on high-level interpreter errors

If we hit an error at this level, we are almost certainly not in any kind
of sane state where autoboot can do the right thing. Instead of letting it
try and probably failing, disable autoboot so they immediately get kicked
into a loader prompt for manual remediation/diagnosis.

Reviewed by: tsoome
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D23611