log_q: Add SystemExit to the ignore list.

This is what will happen if gunicorn receives a SIGTERM (indicating
non-graceful worker shutdown -- likely because the worker has overstayed
its welcome) and so it's not actually useful information.

r2/r2/lib/log.py

@@ -90,6 +90,7 @@ class LogQueueErrorReporter(Reporter):
         import r2.lib.lock
 
         return (
+            SystemExit,  # gunicorn is shutting us down
             _pylibmc.MemcachedError,
             r2.lib.db.thing.NotFound,
             r2.lib.lock.TimeoutExpired,