ui, fw: fixed exception checking fw status

Under certain situation, checking fw status caused an exception
loading the GUI when new nodes connected.

What caused this error is unknown, so for now we'll protect the GUI from
crashing.

Closes #790

ui/opensnitch/dialogs/firewall.py

@@ -230,6 +230,9 @@ def _check_fw_status(self):
                     self._set_status_error(QC.translate("firewall", "Error getting OUTPUT chain policy"))
                     self._disable_widgets()
 
+        except Exception as e:
+            self._set_status_error("Firewall status error (report on github please): {0}".format(e))
+
         finally:
             # some nodes may have the firewall disabled whilst other enabled
             #if not enableFw: