Fix stack trace when no keyring exist

[vincele]

$ ./cyberpower/venv-cyberpower/bin/cyberpower pdu status Traceback (most recent call last):
  File "./cyberpower/venv-cyberpower/bin/cyberpower", line 7, in <module>
    sys.exit(main())
             ~~~~^^
  File "./cyberpower/src/cyberpower/cli.py", line 107, in main
    return do_status(args)
  File "./cyberpower/src/cyberpower/cli.py", line 58, in do_status
    with CyberPower(args.host, args.user) as c:
         ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File "./cyberpower/src/cyberpower/cyberpower.py", line 27, in __init__
    password or keyring.get_password(self.host, self.user) or getpass.getpass()
                ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File "./cyberpower/venv-cyberpower/lib/python3.14/site-packages/keyring/core.py", line 55, in get_password
    return get_keyring().get_password(service_name, username)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^
  File "./cyberpower/venv-cyberpower/lib/python3.14/site-packages/keyring/backends/fail.py", line 25, in get_password
    raise NoKeyringError(msg)