util.CallbackManager: handle callbacks being cancelled

was getting log spam when running the pycharm debugger in certain cases:
```
  3.29 | E | concurrent.futures | exception calling callback for <Future at 0x1d4987bcc70 state=cancelled>
Traceback (most recent call last):
  File "...\Python310\lib\concurrent\futures\_base.py", line 342, in _invoke_callbacks
    callback(self)
  File "...\electrum\electrum\util.py", line 1785, in on_done
    if exc := fut_.exception():
  File "...\Python310\lib\concurrent\futures\_base.py", line 485, in exception
    raise CancelledError()
concurrent.futures._base.CancelledError
```

electrum/util.py

@@ -1782,7 +1782,9 @@ class CallbackManager(Logger):
                 def on_done(fut_: concurrent.futures.Future):
                     assert fut_.done()
                     self._running_cb_futs.remove(fut_)
-                    if exc := fut_.exception():
+                    if fut_.cancelled():
+                        self.logger.debug(f"cb cancelled. {event=}.")
+                    elif exc := fut_.exception():
                         self.logger.error(f"cb errored. {event=}. {exc=}", exc_info=exc)
                 fut.add_done_callback(on_done)
             else:  # non-async cb