OpmlExportJobState model instances set their own "filename" attribute to nil before validation if the "state" attribute has a value different from "SUCCESS".

This ensures that validations on this model do not fail if, e.g. a "SUCCESS" state (which must have a not nil "filename") is updated to "ERROR").
The state of the OPML export process is displayed to the user through alerts in the start page and temporary alerts at the top of the page.

Not yet implemented: allowing the user to download the file.