fix main script hanging (not exiting after exception) in some cases
fix main script hanging (not exiting after exception) in some cases

Previously an unhandled exception in the main script could cause the main thread to die
but the process to hang, as the event loop thread would keep running.

example:
$ ./run_electrum -o signmessage tb1qeh090ruc3cs5hry90tev4fsvrnegulw8xssdzx "mymsg" -w ~/.electrum/testnet/wallets/test_segwit_2
Traceback (most recent call last):
  File "./run_electrum", line 424, in <module>
    init_cmdline(config_options, wallet_path, False)
  File "./run_electrum", line 146, in init_cmdline
    db = WalletDB(storage.read(), manual_upgrades=False)
  File "/home/user/wspace/electrum/electrum/wallet_db.py", line 72, in __init__
    self.load_data(raw)
  File "/home/user/wspace/electrum/electrum/wallet_db.py", line 103, in load_data
    self._after_upgrade_tasks()
  File "/home/user/wspace/electrum/electrum/wallet_db.py", line 189, in _after_upgrade_tasks
    self._load_transactions()
  File "/home/user/wspace/electrum/electrum/util.py", line 406, in <lambda>
    return lambda *args, **kw_args: do_profile(args, kw_args)
  File "/home/user/wspace/electrum/electrum/util.py", line 402, in do_profile
    o = func(*args, **kw_args)
  File "/home/user/wspace/electrum/electrum/wallet_db.py", line 1139, in _load_transactions
    self.data = StoredDict(self.data, self, [])
  File "/home/user/wspace/electrum/electrum/json_db.py", line 79, in __init__
    self.__setitem__(k, v)
  File "/home/user/wspace/electrum/electrum/json_db.py", line 44, in wrapper
    return func(self, *args, **kwargs)
  File "/home/user/wspace/electrum/electrum/json_db.py", line 105, in __setitem__
    v = self.db._convert_dict(self.path, key, v)
  File "/home/user/wspace/electrum/electrum/wallet_db.py", line 1182, in _convert_dict
    v = dict((k, Invoice.from_json(x)) for k, x in v.items())
  File "/home/user/wspace/electrum/electrum/wallet_db.py", line 1182, in <genexpr>
    v = dict((k, Invoice.from_json(x)) for k, x in v.items())
  File "/home/user/wspace/electrum/electrum/invoices.py", line 110, in from_json
    return OnchainInvoice(**x)
  File "<attrs generated init electrum.invoices.OnchainInvoice>", line 8, in __init__
  File "/home/user/wspace/electrum/electrum/invoices.py", line 68, in _decode_outputs
    output = PartialTxOutput.from_legacy_tuple(*output)
  File "/home/user/wspace/electrum/electrum/transaction.py", line 131, in from_legacy_tuple
    return cls.from_address_and_value(addr, val)
  File "/home/user/wspace/electrum/electrum/transaction.py", line 104, in from_address_and_value
    return cls(scriptpubkey=bfh(bitcoin.address_to_script(address)),
  File "/home/user/wspace/electrum/electrum/bitcoin.py", line 422, in address_to_script
    raise BitcoinException(f"invalid bitcoin address: {addr}")
electrum.util.BitcoinException: invalid bitcoin address: tb1qckp4ztmstwtyxzml3dmfvegeq5mfxwu2h3q94l

pooler/electrum-ltc,pooler/electrum-ltc,wakiyamap/electrum-mona,pooler/electrum-ltc,wakiyamap/electrum-mona,wakiyamap/electrum-mona,spesmilo/electrum,spesmilo/electrum,pooler/electrum-ltc,spesmilo/electrum,wakiyamap/electrum-mona,spesmilo/electrum