Improve error message when user fails to enter a username.

[Erotemic]

This is simply a suggestion for a place where a better error message might improve UX.

It took me way too long to debug this issue, but in this recent CI run:

https://github.com/GERSL/pycold/runs/8263806866?check_suite_focus=true

I got an error:

Traceback (most recent call last):
  File "/home/runner/.local/bin/twine", line 8, in <module>
    sys.exit(main())
  File "/home/runner/.local/lib/python3.8/site-packages/twine/__main__.py", line 33, in main
    error = cli.dispatch(sys.argv[1:])
  File "/home/runner/.local/lib/python3.8/site-packages/twine/cli.py", line 123, in dispatch
    return main(args.args)
  File "/home/runner/.local/lib/python3.8/site-packages/twine/commands/upload.py", line 198, in main
    return upload(upload_settings, parsed_args.dists)
  File "/home/runner/.local/lib/python3.8/site-packages/twine/commands/upload.py", line 127, in upload
    repository = upload_settings.create_repository()
  File "/home/runner/.local/lib/python3.8/site-packages/twine/settings.py", line 329, in create_repository
    self.username,
  File "/home/runner/.local/lib/python3.8/site-packages/twine/settings.py", line 131, in username
    return cast(Optional[str], self.auth.username)
  File "/home/runner/.local/lib/python3.8/site-packages/twine/auth.py", line 34, in username
    return utils.get_userpass_value(
  File "/home/runner/.local/lib/python3.8/site-packages/twine/utils.py", line 248, in get_userpass_value
    value = prompt_strategy()
  File "/home/runner/.local/lib/python3.8/site-packages/twine/auth.py", line 85, in username_from_keyring_or_prompt
    return self.prompt("username", input)
  File "/home/runner/.local/lib/python3.8/site-packages/twine/auth.py", line 96, in prompt
    return how(f"Enter your {what}: ")
EOFError: EOF when reading a line
Enter your username: failed to twine upload
Error: Process completed with exit code 1.

And I spent a lot of time focusing on the EOFError: EOF when reading a line part of the issue and I didn't really see the Enter your username:, especially because it was followed by failed to twine upload.

With the new change to enforcing token based credentials, I think it may be more common for users to forget to put --username __token__ when using a token as the password in a non-interactive session. So maybe raising a more clear ValueError indication: "A Username was not specified, please enter a valid username or rerun with --username token if using a token as a password or --username if using your username/password credentials"...

Maybe that's a bit too wordy, but something along those lines might be nice. (It might even be nicer to just have a --token <token> argument as an alias for --username __token__ --password <token>.