It is actually much more obvious than merely a level indicator. At 1000 miles remaining, an yellow notification appears in on the instrument panel clearly counting down the miles remaining before the vehicle will not start:
At 200 miles, the advisory turns red and becomes a warning:
When the countdown reaches 0 miles, the vehicle will no longer start. I don't believe the vehicle will shut down when you reach 0, but I've not seen official word there. You can hardly claim that you had no idea the DEF was low or that you were stranded unknowingly in a remote area. If you fail to heed the notification and the warning, you've only yourself to blame.