Fix exception when clicking on "Donate to server" in offline mode by not
showing the "Donate to server" menu action when no network is set.
Raise CancelledError in `fetch_bitcoin_paper` as
`_fetch_tx_from_network` already shows an according error message so the
second, subsequent error message shown by `on_error` is not useful if
`_fetch_tx_from_network` already failed.
```
63.53 | E | gui.qt.exception_window.Exception_Hook | exception caught by crash reporter
Traceback (most recent call last):
File "/home/user/Documents/electrum/electrum/gui/qt/main_window.py", line 864, in donate_to_server
d = self.network.get_donation_address()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'get_donation_address'
```
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