utils/openqa_junit_report.py: Fix issue and improve docs
Fixes error seen here: https://gitlab.gnome.org/GNOME/gnome-build-meta/-/jobs/2814806
Traceback (most recent call last):
File "openqa/utils/openqa_junit_report.py", line 148, in <module>
main()
File "openqa/utils/openqa_junit_report.py", line 145, in main
writer.generate_xml(sys.stdout, junit_test_results)
File "openqa/utils/openqa_junit_report.py", line 62, in generate_xml
failure.set('type', result['failure']['type'])
KeyError: 'type'