Make the plugin portal test double require qualified plugin ids, because the real one does.
Improve error messages when a plugin fails to apply.

+review REVIEW-5046
Improve error messages when a plugin fails to apply (cont.)

+review REVIEW-5046
Break out test case from story that is too much work to implement in this round of work.

- Added open issue on this story and added later story to address.

+review REVIEW-5046