cli: improve exception handling for erroneous configuration

[virtualtam]

Fixes https://github.com/shaarli/python-shaarli-client/issues/19

See:

• https://docs.python.org/3.5/library/configparser.html
• https://docs.pytest.org/en/latest/parametrize.html
• https://docs.pytest.org/en/latest/tmpdir.html
• https://py.readthedocs.io/en/latest/path.html