Report error instead of stub XML for non-HFS disk images

[dd388]

Test images with no HFS partition and verify that an error gets written out instead of a stub XML file reporting an HFS volume.

[dd388]

Stubs were created because there is no error attribute in a VolumeObject. Went back to the original functioning, where if there is no HFS partition the whole script quits.
801da91

However, it might be better to have a VolumeObject report an error instead. Leaving this open for now.

[ajnelson-nist]

VolumeObject.error should now work, as of 7cee035. Thanks for raising this issue.