WebSocket receive error: cancel_wait_ex: [Errno 9] File descriptor was closed in another greenlet

### Step 1: Please describe your environment

* ZeroNet version: r4473
* Operating system: W10
* Web browser: FF beta

### Step 2: Describe the problem:

i have seen several errors in the debug.log that appeared 6 seconds before "INFO - All server stopped":

> ERROR Site:1MeFqF..q7nH WebSocket receive error: cancel_wait_ex: [Errno 9] File descriptor was closed in another greenlet in core/src/Ui/UiWebsocket.py line 71 > ... > _hub_primitives.py line 284 > 289 > 271 > 46 > 46 > 55 > _waiter.py line 151 > _greenlet_primitives.py line 60 > 60 > 64 > __greenlet_primitives.pxd line 35
> ERROR Site:1ADQAH..g6aA WebSocket receive error: cancel_wait_ex: [Errno 9] File descriptor was closed in another greenlet in core/src/Ui/UiWebsocket.py line 71 > ... > _hub_primitives.py line 284 > 289 > 271 > 46 > 46 > 55 > _waiter.py line 151 > _greenlet_primitives.py line 60 > 60 > 64 > __greenlet_primitives.pxd line 35

later Windows reported that zeronet process stopped working and invited to kill it (instead of clean shutdown during update from the updater zite - update was not available on the zite, i wanted just restart)

Start with core/src/Ui/UiWebsocket.py at line 71 and compare the reported debug.log errors with the shutdown sequence that ends in "All server stopped". Reproduce or trace the Windows restart/update path, then confirm that shutdown no longer produces the file-descriptor error or leaves the ZeroNet process stuck.

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