The convention is that sys.exit(...) is called only if a module is run as a script,
and that main() entry points use return statements to report exit codes: 0 (or None)
for successful execution, or any non-zero integer for errors.
E.g. see the console scripts generated when installing with pip.
In the OpenType 1.8 specification, this is called TupleVariation
so let's be consistent with the spec. (The initial implementation
in fonttools pre-dates OpenType 1.8).
Here I use logging.basicConfig to configure the root logger, and hence all the other loggers,
including fontTools'.
I could have used fontTools.configLogger as well (and get predefined formatter, etc.), but wanted
to show that one can also configure the fontTools logger using the built-in logging configuration
functions (e.g. basicConfig, config.dictConfig and config.fileConfig).
With this change, the "Thin" instance now renders fine, too.
Apparently, the axis values need to be positive; this was not
clear to me from reading the specification.
