Improve handling of missing config file

[bhrutledge]

Fixes #564

There's a lot here, but my hope is that it makes things clearer for users and contributors. I tried to organize the commits so that they could be reviewed individually; for example, the change that addresses @angerson's suggestion in #564 (comment) is fd51ed1.

New output:

% twine upload --config-file /path/to/missing/file --repository undefined-repository dist/* 
InvalidConfiguration: [Errno 2] No such file or directory: '/path/to/missing/file'

% twine upload --config-file /path/to/missing/file dist/*
InvalidConfiguration: [Errno 2] No such file or directory: '/path/to/missing/file'

% twine upload --repository undefined-repository dist/*                      
InvalidConfiguration: Missing 'undefined-repository' section from ~/.pypirc.
More info: https://packaging.python.org/specifications/pypirc/ 

The second example is new behavior. Currently, Twine silently ignores the missing file, and returns a default configuration for PyPI and TestPyPI. However, if a user takes the time to specify --config-file on the command line, and the file doesn't exist, that feels like it should be an error. These changes include a special case to continue to provide a default configuration if --config-file isn't used.

[bhrutledge]

@pypa/twine-maintainers Friendly nudge for some feedback. 😅