Hub: better handle_system_error

make sure we deliver exception to MAIN greenlet even if loop.run_callback fails

gevent/hub.py

@@ -320,11 +320,16 @@ class Hub(greenlet):
         else:
             # in case system error was handled and life goes on
             # switch back to this greenlet as well
-            cb = self.loop.run_callback(current.switch)
+            cb = None
+            try:
+                cb = self.loop.run_callback(current.switch)
+            except:
+                traceback.print_exc()
             try:
                 self.parent.throw(type, value)
             finally:
-                cb.stop()
+                if cb is not None:
+                    cb.stop()
 
     def print_exception(self, context, type, value, tb):
         traceback.print_exception(type, value, tb)