Improve feedback when loading data from CMS

[brew]

Currently, the various admin actions for loading data from the CMS return a JSON snippet in the body of the response.

Often, if there is a problem with the input data that causes an error, the error message isn't detailed enough to help admins fix the problem.

This can be improved by returning the admin page with a flash message indicating success/failure of the action, and return an improved error message to help admins fix problems.

• validate Question Set schema against json schema
• return admin page, rather than json snippet
• display flash message for success/failure with error details