fallback to importing index.cnxml

[bdolor]

Description

Some zip files do not contain index.cnxml.html files - they only have index.cnxml

Expected behaviour

While not as straight forward, the Importer should at least try and bring something in from the xml file.

Actual behaviour

An exception is thrown and it imports a blank page.

Steps to reproduce the problem

try importing any one (or more) chapters from https://cnx.org/contents/MDgA8wfz@22.2:YzfkjC2r@17/

System Information

• Pressbooks OpenStax Import: [0.1.3]
• Pressbooks: [5.0.0-beta]
• WordPress: [4.9.4]