Add a default error code when updater script aborts

We didn't report error/cause codes unless there's an explict "Abort()"
call inside the updater script. As a result, some cause codes set by
ErrorAbort() didn't show up in last_install.

To fix the issue, add a default error code when the script terminates
abnormally (i.e. with non zero status).

Bug: 37912405
Test: error/cause code shows up in last_install when argument parsing fails
Change-Id: Ic6d3bd1855b853aeaa0760071e593a00cf6f0209