Fix cleaning up event unsubscription maintenance

Closes L2SS-1258

tangostationcontrol/tangostationcontrol/devices/temperature_manager.py

@@ -137,9 +137,16 @@ class TemperatureManager(LOFARDevice):
         super().configure_for_off()
 
         # unsubscribe from all events
-        for dev_attr in self.temp_error_attrs.values():
-            dev_attr.proxy.unsubscribe_event(dev_attr.subscription_id)
-            del dev_attr
+        for attr_name in list(self.temp_error_attrs.keys()):
+            try:
+                dev_attr = self.temp_error_attrs[attr_name]
+                dev_attr.proxy.unsubscribe_event(dev_attr.subscription_id)
+            except Exception as e:
+                raise Exception(
+                    f"Failed to unsubscribe from event regarding device {dev_attr.proxy.dev_name()} attribute {attr_name}"
+                ) from e
+            finally:
+                del self.temp_error_attrs[attr_name]
 
     is_alarming_R = attribute(
         dtype=bool,