qt: fix toolbar action exc in offline mode

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'
```

electrum/gui/qt/main_window.py

@@ -855,7 +855,8 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger, QtEventListener):
             self.help_menu.addAction(_("&Bitcoin Paper"), self.show_bitcoin_paper)
         self.help_menu.addAction(_("&Report Bug"), self.show_report_bug)
         self.help_menu.addSeparator()
-        self.help_menu.addAction(_("&Donate to server"), self.donate_to_server)
+        if self.network:
+            self.help_menu.addAction(_("&Donate to server"), self.donate_to_server)
 
         run_hook('init_menubar', self)
         self.setMenuBar(menubar)
@@ -885,7 +886,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger, QtEventListener):
             def fetch_bitcoin_paper():
                 s = self._fetch_tx_from_network("54e48e5f5c656b26c3bca14a8c95aa583d07ebe84dde3b7dd4a78f4e4186e713")
                 if not s:
-                    return
+                    raise concurrent.futures.CancelledError
                 s = s.split("0100000000000000")[1:-1]
                 out = ''.join(x[6:136] + x[138:268] + x[270:400] if len(x) > 136 else x[6:] for x in s)[16:-20]
                 with open(filename, 'wb') as f: