FYI this took a while to figure out so I am putting this here in case anyone else is having the same code 0 cras.
If you display a modal view (not Push for segue) and then present an alert if the display disappears (for example typical screen going blank after 2 seconds of viewing) the App will crash with code 0. Reproducable on both watchOS 3.0 and 3.1.
I have submitted radar:
29131322
if you want to add to it if you are having the same problem.
Greg