In trying to import an Angel-exported IMS Common Cartridge, I received an error "The schema is not valid."
I see several similar issues here, most closed with "not a bug"
Testing with IMS Desktop Validator, I found that, indeed, Angel generates broken cartridges.
But downloading and installing and validating took a fair amount of effort, and I would have preferred Moodle to have just told me specifically what was broken instead.
I therefore humbly submit my first patch, which organizes import errors by level and dumps them out as notify messages. It's a bit verbose, and perhaps should be an expandable display: none; div instead, but it's a start. I don't know enough Moodle internals yet to do that properly, and I doubt you'd want inline JS hackery in the patch...
I added a CSS class for level, but did not actually style them in any particular way; I leave that to somebody more familiar with Moodle front-end.
There is not much I can do to convince Blackboard to actually fix Angel IMS CC export; But at least I can help Moodlers know that the cartridge they are trying to import is truly broken, and exactly how.
This also helps nudge Moodle closer to IMS CC compliance, which specifically states:
"On import LMS should confirm that package has imported properly or that there were errors importing the package. Error messages should attempt to direct the user to the problem within the package such as identify a resource that could not be located within the package." section 6.2.4 of imscc_profilev1p0.