Arvados

By default, Python exits 1 when there's any uncaught exception. We should introspect these for exceptions that represent temporary failures (KeepReadError but not its subclasses, KeepWriteError), and arrange to exit 111 when they're uncaught, to signal to Crunch that the task can be retried.

We can do this by defining sys.excepthook, although it will take a little care to avoid interfering with other scripts that set this. Suggest:

def arvados_excepthook(exc_type, exc_value, traceback):
    # Write traceback, do introspection, exit here.

if sys.excepthook is sys.__excepthook__:
    sys.excepthook = arvados_excepthook