wallet: is_up_to_date() to return False if taskgroup stopped

If the taskgroup died unexpectedly, this will result in the GUI
showing we are in the "synchronizing" state instead of the green orb.
Being stuck in "synchronizing" provides at least *some* feedback to
the user that something is wrong.

see https://github.com/spesmilo/electrum/issues/8301

electrum/wallet.py

@@ -373,6 +373,7 @@ class Abstract_Wallet(ABC, Logger, EventListener):
         except Exception as e:
             self.logger.exception("taskgroup died.")
         finally:
+            util.trigger_callback('wallet_updated', self)
             self.logger.info("taskgroup stopped.")
 
     async def do_synchronize_loop(self):
@@ -460,6 +461,8 @@ class Abstract_Wallet(ABC, Logger, EventListener):
             self.save_db()
 
     def is_up_to_date(self) -> bool:
+        if self.taskgroup.joined:  # either stop() was called, or the taskgroup died
+            return False
         return self._up_to_date
 
     def tx_is_related(self, tx):