when `debug', abort when lisp error during loadup
loadup.el: When featurep `debug-xemacs' (configure --with-debug), set
debug-on-error, so that we get an exit-to-debugger/assertion
failure upon Lisp error during loadup. Unset before dumping.
cmdloop.c: During really-early-error-handler, exit to the debugger and abort
if an error occurs and
lisp.h: extern Vdebug_on_error.