Make "This machines is too slow ..." non-fatal.

This failure may be intermittent and it currently stops the
worker.

worker/games.py

@@ -1372,7 +1372,7 @@ def parse_options(s):
     )
 
     if base_nps < 231000 / (1 + math.tanh((worker_concurrency - 1) / 8)):
-        raise FatalException(
+        raise WorkerException(
             "This machine is too slow ({} nps / thread) to run fishtest effectively - sorry!".format(
                 base_nps
             )