prove-all: do not exit with 0 when rejected specs have been cached

[asymmetric]

Klab caches rejected as well as accepted specs, meaning that if running prove-all fails due to some specs being rejected, the next run (assuming no specs have changed) will return 0, which is the status code for "no failing specs".

One way of doing this could be to check for matching rejected specs when deciding the status code to return.