core: ensure 'exit' signal fires even on Broker crash.

mitogen/core.py

@@ -3272,7 +3272,10 @@ class Broker(object):
         self._broker_exit()
 
     def _broker_main(self):
+        try:
             _profile_hook('mitogen.broker', self._do_broker_main)
+        finally:
+            # 'finally' to ensure _on_broker_exit() can always SIGTERM.
             fire(self, 'exit')
 
     def shutdown(self):