fixes https://github.com/spesmilo/electrum/issues/7447
Consider this trace for 4.2.0:
```
Traceback (most recent call last):
File "electrum/gui/qt/__init__.py", line 332, in start_new_window
File "electrum/gui/qt/__init__.py", line 363, in _start_wizard_to_select_or_create_wallet
File "electrum/gui/qt/installwizard.py", line 302, in select_storage
File "electrum/util.py", line 504, in get_new_wallet_name
PermissionError: [Errno 1] Operation not permitted: '/Users/admin/Documents/Peach/MS'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "electrum/gui/qt/__init__.py", line 426, in main
File "electrum/gui/qt/__init__.py", line 307, in wrapper
File "electrum/gui/qt/__init__.py", line 349, in start_new_window
File "electrum/util.py", line 504, in get_new_wallet_name
PermissionError: [Errno 1] Operation not permitted: '/Users/admin/Documents/Peach/MS'
```
Note that `get_new_wallet_name` (os.listdir) can raise OSError,
and we were calling that on the main entrypoint codepath without exception-handling.
We were also calling it in the fallback codepath without exception-handling.
i.e. the GUI errored out on every startup for affected users, and without CLI usage
it was not possible to recover.
05649861c8