Tiago Gomes authored Aug 16, 2018 and Javier Jardón committed Aug 16, 2018

os.errno does no longer work with Python 3.7

Closes #577.

cascache: use errno module

See merge request !675

Raise a error at configure time if the track and ref properties are
not present in the sources.

This is to address #471
that documented unhelpful behaviour when tracking git sources. However
the issue was also identified in ostree.